diff --git a/src/net/http/request.go b/src/net/http/request.go
index c29af7fbe5..83d6c81de9 100644
--- a/src/net/http/request.go
+++ b/src/net/http/request.go
@@ -742,6 +742,15 @@ func NewRequest(method, urlStr string, body io.Reader) (*Request, error) {
 			req.ContentLength = int64(v.Len())
 		case *strings.Reader:
 			req.ContentLength = int64(v.Len())
+		default:
+			req.ContentLength = -1 // unknown
+		}
+		// For client requests, Request.ContentLength of 0
+		// means either actually 0, or unknown. The only way
+		// to explicitly say that the ContentLength is zero is
+		// to set the Body to nil.
+		if req.ContentLength == 0 {
+			req.Body = nil
 		}
 	}
 
@@ -1216,49 +1225,14 @@ func (r *Request) isReplayable() bool {
 	return false
 }
 
-// bodyAndLength reports the request's body and content length, with
-// the difference from r.ContentLength being that 0 means actually
-// zero, and -1 means unknown.
-func (r *Request) bodyAndLength() (body io.Reader, contentLen int64) {
-	body = r.Body
-	if body == nil {
-		return nil, 0
+// outgoingLength reports the Content-Length of this outgoing (Client) request.
+// It maps 0 into -1 (unknown) when the Body is non-nil.
+func (r *Request) outgoingLength() int64 {
+	if r.Body == nil {
+		return 0
 	}
 	if r.ContentLength != 0 {
-		return body, r.ContentLength
+		return r.ContentLength
 	}
-
-	// Don't try to sniff the request body if,
-	// * they're using a custom transfer encoding (or specified
-	//   chunked themselves)
-	// * they're not using HTTP/1.1 and can't chunk anyway (even
-	//   though this is basically irrelevant, since this package
-	//   only sends minimum 1.1 requests)
-	// * they're sending an "Expect: 100-continue" request, because
-	//   they might get denied or redirected and try to use the same
-	//   body elsewhere, so we shoudn't consume it.
-	if len(r.TransferEncoding) != 0 ||
-		!r.ProtoAtLeast(1, 1) ||
-		r.Header.Get("Expect") == "100-continue" {
-		return body, -1
-	}
-
-	// Test to see if it's actually zero or just unset.
-	var buf [1]byte
-	n, err := io.ReadFull(body, buf[:])
-	if err != nil && err != io.EOF {
-		return errorReader{err}, -1
-	}
-
-	if n == 1 {
-		// Oh, guess there is data in this Body Reader after all.
-		// The ContentLength field just wasn't set.
-		// Stich the Body back together again, re-attaching our
-		// consumed byte.
-		// TODO(bradfitz): switch to stitchByteAndReader
-		return io.MultiReader(bytes.NewReader(buf[:]), body), -1
-	}
-
-	// Body is actually empty.
-	return nil, 0
+	return -1
 }
diff --git a/src/net/http/request_test.go b/src/net/http/request_test.go
index c52eb81f03..f12b41cf1b 100644
--- a/src/net/http/request_test.go
+++ b/src/net/http/request_test.go
@@ -497,18 +497,22 @@ func TestNewRequestContentLength(t *testing.T) {
 		{bytes.NewReader([]byte("123")), 3},
 		{bytes.NewBuffer([]byte("1234")), 4},
 		{strings.NewReader("12345"), 5},
+		{strings.NewReader(""), 0},
 		// Not detected:
-		{struct{ io.Reader }{strings.NewReader("xyz")}, 0},
-		{io.NewSectionReader(strings.NewReader("x"), 0, 6), 0},
-		{readByte(io.NewSectionReader(strings.NewReader("xy"), 0, 6)), 0},
+		{struct{ io.Reader }{strings.NewReader("xyz")}, -1},
+		{io.NewSectionReader(strings.NewReader("x"), 0, 6), -1},
+		{readByte(io.NewSectionReader(strings.NewReader("xy"), 0, 6)), -1},
 	}
-	for _, tt := range tests {
+	for i, tt := range tests {
 		req, err := NewRequest("POST", "http://localhost/", tt.r)
 		if err != nil {
 			t.Fatal(err)
 		}
 		if req.ContentLength != tt.want {
-			t.Errorf("ContentLength(%T) = %d; want %d", tt.r, req.ContentLength, tt.want)
+			t.Errorf("test[%d]: ContentLength(%T) = %d; want %d", i, tt.r, req.ContentLength, tt.want)
+		}
+		if (req.ContentLength == 0) != (req.Body == nil) {
+			t.Errorf("test[%d]: ContentLength = %d but Body non-nil is %v", i, req.ContentLength, req.Body != nil)
 		}
 	}
 }
@@ -667,11 +671,31 @@ func TestStarRequest(t *testing.T) {
 	if err != nil {
 		return
 	}
+	if req.ContentLength != 0 {
+		t.Errorf("ContentLength = %d; want 0", req.ContentLength)
+	}
+	if req.Body == nil {
+		t.Errorf("Body = nil; want non-nil")
+	}
+
+	// Request.Write has Client semantics for Body/ContentLength,
+	// where ContentLength 0 means unknown if Body is non-nil, and
+	// thus chunking will happen unless we change semantics and
+	// signal that we want to serialize it as exactly zero.  The
+	// only way to do that for outbound requests is with a nil
+	// Body:
+	clientReq := *req
+	clientReq.Body = nil
+
 	var out bytes.Buffer
-	if err := req.Write(&out); err != nil {
+	if err := clientReq.Write(&out); err != nil {
 		t.Fatal(err)
 	}
-	back, err := ReadRequest(bufio.NewReader(&out))
+
+	if strings.Contains(out.String(), "chunked") {
+		t.Error("wrote chunked request; want no body")
+	}
+	back, err := ReadRequest(bufio.NewReader(bytes.NewReader(out.Bytes())))
 	if err != nil {
 		t.Fatal(err)
 	}
diff --git a/src/net/http/requestwrite_test.go b/src/net/http/requestwrite_test.go
index 2545f6f4c2..d13e37aba0 100644
--- a/src/net/http/requestwrite_test.go
+++ b/src/net/http/requestwrite_test.go
@@ -28,7 +28,7 @@ type reqWriteTest struct {
 
 var reqWriteTests = []reqWriteTest{
 	// HTTP/1.1 => chunked coding; no body; no trailer
-	{
+	0: {
 		Req: Request{
 			Method: "GET",
 			URL: &url.URL{
@@ -75,7 +75,7 @@ var reqWriteTests = []reqWriteTest{
 			"Proxy-Connection: keep-alive\r\n\r\n",
 	},
 	// HTTP/1.1 => chunked coding; body; empty trailer
-	{
+	1: {
 		Req: Request{
 			Method: "GET",
 			URL: &url.URL{
@@ -104,7 +104,7 @@ var reqWriteTests = []reqWriteTest{
 			chunk("abcdef") + chunk(""),
 	},
 	// HTTP/1.1 POST => chunked coding; body; empty trailer
-	{
+	2: {
 		Req: Request{
 			Method: "POST",
 			URL: &url.URL{
@@ -137,7 +137,7 @@ var reqWriteTests = []reqWriteTest{
 	},
 
 	// HTTP/1.1 POST with Content-Length, no chunking
-	{
+	3: {
 		Req: Request{
 			Method: "POST",
 			URL: &url.URL{
@@ -172,7 +172,7 @@ var reqWriteTests = []reqWriteTest{
 	},
 
 	// HTTP/1.1 POST with Content-Length in headers
-	{
+	4: {
 		Req: Request{
 			Method: "POST",
 			URL:    mustParseURL("http://example.com/"),
@@ -201,7 +201,7 @@ var reqWriteTests = []reqWriteTest{
 	},
 
 	// default to HTTP/1.1
-	{
+	5: {
 		Req: Request{
 			Method: "GET",
 			URL:    mustParseURL("/search"),
@@ -215,7 +215,7 @@ var reqWriteTests = []reqWriteTest{
 	},
 
 	// Request with a 0 ContentLength and a 0 byte body.
-	{
+	6: {
 		Req: Request{
 			Method:        "POST",
 			URL:           mustParseURL("/"),
@@ -227,9 +227,32 @@ var reqWriteTests = []reqWriteTest{
 
 		Body: func() io.ReadCloser { return ioutil.NopCloser(io.LimitReader(strings.NewReader("xx"), 0)) },
 
-		// RFC 2616 Section 14.13 says Content-Length should be specified
-		// unless body is prohibited by the request method.
-		// Also, nginx expects it for POST and PUT.
+		WantWrite: "POST / HTTP/1.1\r\n" +
+			"Host: example.com\r\n" +
+			"User-Agent: Go-http-client/1.1\r\n" +
+			"Transfer-Encoding: chunked\r\n" +
+			"\r\n0\r\n\r\n",
+
+		WantProxy: "POST / HTTP/1.1\r\n" +
+			"Host: example.com\r\n" +
+			"User-Agent: Go-http-client/1.1\r\n" +
+			"Transfer-Encoding: chunked\r\n" +
+			"\r\n0\r\n\r\n",
+	},
+
+	// Request with a 0 ContentLength and a nil body.
+	7: {
+		Req: Request{
+			Method:        "POST",
+			URL:           mustParseURL("/"),
+			Host:          "example.com",
+			ProtoMajor:    1,
+			ProtoMinor:    1,
+			ContentLength: 0, // as if unset by user
+		},
+
+		Body: func() io.ReadCloser { return nil },
+
 		WantWrite: "POST / HTTP/1.1\r\n" +
 			"Host: example.com\r\n" +
 			"User-Agent: Go-http-client/1.1\r\n" +
@@ -244,7 +267,7 @@ var reqWriteTests = []reqWriteTest{
 	},
 
 	// Request with a 0 ContentLength and a 1 byte body.
-	{
+	8: {
 		Req: Request{
 			Method:        "POST",
 			URL:           mustParseURL("/"),
@@ -270,7 +293,7 @@ var reqWriteTests = []reqWriteTest{
 	},
 
 	// Request with a ContentLength of 10 but a 5 byte body.
-	{
+	9: {
 		Req: Request{
 			Method:        "POST",
 			URL:           mustParseURL("/"),
@@ -284,7 +307,7 @@ var reqWriteTests = []reqWriteTest{
 	},
 
 	// Request with a ContentLength of 4 but an 8 byte body.
-	{
+	10: {
 		Req: Request{
 			Method:        "POST",
 			URL:           mustParseURL("/"),
@@ -298,7 +321,7 @@ var reqWriteTests = []reqWriteTest{
 	},
 
 	// Request with a 5 ContentLength and nil body.
-	{
+	11: {
 		Req: Request{
 			Method:        "POST",
 			URL:           mustParseURL("/"),
@@ -311,7 +334,7 @@ var reqWriteTests = []reqWriteTest{
 	},
 
 	// Request with a 0 ContentLength and a body with 1 byte content and an error.
-	{
+	12: {
 		Req: Request{
 			Method:        "POST",
 			URL:           mustParseURL("/"),
@@ -331,7 +354,7 @@ var reqWriteTests = []reqWriteTest{
 	},
 
 	// Request with a 0 ContentLength and a body without content and an error.
-	{
+	13: {
 		Req: Request{
 			Method:        "POST",
 			URL:           mustParseURL("/"),
@@ -352,7 +375,7 @@ var reqWriteTests = []reqWriteTest{
 
 	// Verify that DumpRequest preserves the HTTP version number, doesn't add a Host,
 	// and doesn't add a User-Agent.
-	{
+	14: {
 		Req: Request{
 			Method:     "GET",
 			URL:        mustParseURL("/foo"),
@@ -373,7 +396,7 @@ var reqWriteTests = []reqWriteTest{
 	// an empty Host header, and don't use
 	// Request.Header["Host"]. This is just testing that
 	// we don't change Go 1.0 behavior.
-	{
+	15: {
 		Req: Request{
 			Method: "GET",
 			Host:   "",
@@ -395,7 +418,7 @@ var reqWriteTests = []reqWriteTest{
 	},
 
 	// Opaque test #1 from golang.org/issue/4860
-	{
+	16: {
 		Req: Request{
 			Method: "GET",
 			URL: &url.URL{
@@ -414,7 +437,7 @@ var reqWriteTests = []reqWriteTest{
 	},
 
 	// Opaque test #2 from golang.org/issue/4860
-	{
+	17: {
 		Req: Request{
 			Method: "GET",
 			URL: &url.URL{
@@ -433,7 +456,7 @@ var reqWriteTests = []reqWriteTest{
 	},
 
 	// Testing custom case in header keys. Issue 5022.
-	{
+	18: {
 		Req: Request{
 			Method: "GET",
 			URL: &url.URL{
@@ -457,7 +480,7 @@ var reqWriteTests = []reqWriteTest{
 	},
 
 	// Request with host header field; IPv6 address with zone identifier
-	{
+	19: {
 		Req: Request{
 			Method: "GET",
 			URL: &url.URL{
@@ -472,7 +495,7 @@ var reqWriteTests = []reqWriteTest{
 	},
 
 	// Request with optional host header field; IPv6 address with zone identifier
-	{
+	20: {
 		Req: Request{
 			Method: "GET",
 			URL: &url.URL{
@@ -553,14 +576,14 @@ func (rc *closeChecker) Close() error {
 	return nil
 }
 
-// TestRequestWriteClosesBody tests that Request.Write does close its request.Body.
+// TestRequestWriteClosesBody tests that Request.Write closes its request.Body.
 // It also indirectly tests NewRequest and that it doesn't wrap an existing Closer
 // inside a NopCloser, and that it serializes it correctly.
 func TestRequestWriteClosesBody(t *testing.T) {
 	rc := &closeChecker{Reader: strings.NewReader("my body")}
 	req, _ := NewRequest("POST", "http://foo.com/", rc)
-	if req.ContentLength != 0 {
-		t.Errorf("got req.ContentLength %d, want 0", req.ContentLength)
+	if req.ContentLength != -1 {
+		t.Errorf("got req.ContentLength %d, want -1", req.ContentLength)
 	}
 	buf := new(bytes.Buffer)
 	req.Write(buf)
@@ -571,12 +594,7 @@ func TestRequestWriteClosesBody(t *testing.T) {
 		"Host: foo.com\r\n" +
 		"User-Agent: Go-http-client/1.1\r\n" +
 		"Transfer-Encoding: chunked\r\n\r\n" +
-		// TODO: currently we don't buffer before chunking, so we get a
-		// single "m" chunk before the other chunks, as this was the 1-byte
-		// read from our MultiReader where we stitched the Body back together
-		// after sniffing whether the Body was 0 bytes or not.
-		chunk("m") +
-		chunk("y body") +
+		chunk("my body") +
 		chunk("")
 	if buf.String() != expected {
 		t.Errorf("write:\n got: %s\nwant: %s", buf.String(), expected)
diff --git a/src/net/http/transfer.go b/src/net/http/transfer.go
index b6446486ee..f34c703110 100644
--- a/src/net/http/transfer.go
+++ b/src/net/http/transfer.go
@@ -64,7 +64,8 @@ func newTransferWriter(r interface{}) (t *transferWriter, err error) {
 		t.Trailer = rr.Trailer
 		atLeastHTTP11 = rr.ProtoAtLeast(1, 1)
 
-		t.Body, t.ContentLength = rr.bodyAndLength()
+		t.Body = rr.Body
+		t.ContentLength = rr.outgoingLength()
 		if t.Body != nil {
 			t.BodyCloser = rr.Body
 		}
diff --git a/src/net/http/transport_test.go b/src/net/http/transport_test.go
index f31c858f6b..cef2acc456 100644
--- a/src/net/http/transport_test.go
+++ b/src/net/http/transport_test.go
@@ -2904,14 +2904,8 @@ func TestTransportFlushesBodyChunks(t *testing.T) {
 	defer res.Body.Close()
 
 	want := []string{
-		// Because Request.ContentLength = 0, the body is sniffed for 1 byte to determine whether there's content.
-		// That explains the initial "num0" being split into "n" and "um0".
-		// The first byte is included with the request headers Write. Perhaps in the future
-		// we will want to flush the headers out early if the first byte of the request body is
-		// taking a long time to arrive. But not yet.
 		"POST / HTTP/1.1\r\nHost: localhost:8080\r\nUser-Agent: x\r\nTransfer-Encoding: chunked\r\nAccept-Encoding: gzip\r\n\r\n" +
-			"1\r\nn\r\n",
-		"4\r\num0\n\r\n",
+			"5\r\nnum0\n\r\n",
 		"5\r\nnum1\n\r\n",
 		"5\r\nnum2\n\r\n",
 		"0\r\n\r\n",