diff --git a/src/runtime/mgcmark.go b/src/runtime/mgcmark.go
index dfddd8c6f6..cbdf2b8375 100644
--- a/src/runtime/mgcmark.go
+++ b/src/runtime/mgcmark.go
@@ -601,7 +601,13 @@ func gcFlushBgCredit(scanWork int64) {
 			gp.gcAssistBytes = 0
 			xgp := gp
 			gp = gp.schedlink.ptr()
-			ready(xgp, 0, true)
+			// It's important that we *not* put xgp in
+			// runnext. Otherwise, it's possible for user
+			// code to exploit the GC worker's high
+			// scheduler priority to get itself always run
+			// before other goroutines and always in the
+			// fresh quantum started by GC.
+			ready(xgp, 0, false)
 		} else {
 			// Partially satisfy this assist.
 			gp.gcAssistBytes += scanBytes
diff --git a/src/runtime/proc_test.go b/src/runtime/proc_test.go
index 8994121071..22e4dca771 100644
--- a/src/runtime/proc_test.go
+++ b/src/runtime/proc_test.go
@@ -344,6 +344,14 @@ func TestGCFairness(t *testing.T) {
 	}
 }
 
+func TestGCFairness2(t *testing.T) {
+	output := runTestProg(t, "testprog", "GCFairness2")
+	want := "OK\n"
+	if output != want {
+		t.Fatalf("want %s, got %s\n", want, output)
+	}
+}
+
 func TestNumGoroutine(t *testing.T) {
 	output := runTestProg(t, "testprog", "NumGoroutine")
 	want := "1\n"
diff --git a/src/runtime/testdata/testprog/gc.go b/src/runtime/testdata/testprog/gc.go
index 0676e9a4ec..a0c1f82b56 100644
--- a/src/runtime/testdata/testprog/gc.go
+++ b/src/runtime/testdata/testprog/gc.go
@@ -8,11 +8,14 @@ import (
 	"fmt"
 	"os"
 	"runtime"
+	"runtime/debug"
+	"sync/atomic"
 	"time"
 )
 
 func init() {
 	register("GCFairness", GCFairness)
+	register("GCFairness2", GCFairness2)
 	register("GCSys", GCSys)
 }
 
@@ -72,3 +75,35 @@ func GCFairness() {
 	time.Sleep(10 * time.Millisecond)
 	fmt.Println("OK")
 }
+
+func GCFairness2() {
+	// Make sure user code can't exploit the GC's high priority
+	// scheduling to make scheduling of user code unfair. See
+	// issue #15706.
+	runtime.GOMAXPROCS(1)
+	debug.SetGCPercent(1)
+	var count [3]int64
+	var sink [3]interface{}
+	for i := range count {
+		go func(i int) {
+			for {
+				sink[i] = make([]byte, 1024)
+				atomic.AddInt64(&count[i], 1)
+			}
+		}(i)
+	}
+	// Note: If the unfairness is really bad, it may not even get
+	// past the sleep.
+	//
+	// If the scheduling rules change, this may not be enough time
+	// to let all goroutines run, but for now we cycle through
+	// them rapidly.
+	time.Sleep(30 * time.Millisecond)
+	for i := range count {
+		if atomic.LoadInt64(&count[i]) == 0 {
+			fmt.Printf("goroutine %d did not run\n", i)
+			return
+		}
+	}
+	fmt.Println("OK")
+}