diff --git a/client_test.go b/client_test.go
index 890385c..a4c8ea7 100644
--- a/client_test.go
+++ b/client_test.go
@@ -4,6 +4,7 @@ import (
 	"bufio"
 	"bytes"
 	"errors"
+	"fmt"
 	"io"
 	"log"
 	"net"
@@ -712,12 +713,12 @@ func TestClientRetransmission(t *testing.T) {
 	agent.start = func(id [TransactionIDSize]byte, deadline time.Time) error {
 		if attempt == 0 {
 			attempt++
-			agent.h(Event{
+			go agent.h(Event{
 				TransactionID: id,
 				Error:         ErrTransactionTimeOut,
 			})
 		} else {
-			agent.h(Event{
+			go agent.h(Event{
 				TransactionID: id,
 				Message:       response,
 			})
@@ -734,6 +735,7 @@ func TestClientRetransmission(t *testing.T) {
 		t.Fatal(err)
 	}
 	c.SetRTO(time.Second)
+	gotReads := make(chan struct{})
 	go func() {
 		buf := make([]byte, 1500)
 		readN, readErr := connL.Read(buf)
@@ -750,6 +752,7 @@ func TestClientRetransmission(t *testing.T) {
 		if !IsMessage(buf[:readN]) {
 			t.Error("should be STUN")
 		}
+		gotReads <- struct{}{}
 	}()
 	if doErr := c.Do(MustBuild(response, BindingRequest), func(event Event) {
 		if event.Error != nil {
@@ -758,5 +761,81 @@ func TestClientRetransmission(t *testing.T) {
 	}); doErr != nil {
 		t.Fatal(err)
 	}
+	<-gotReads
+}
+
+func testClientDoConcurrent(t *testing.T, concurrency int) {
+	response := MustBuild(TransactionID, BindingSuccess)
+	response.Encode()
+	connL, connR := net.Pipe()
+	defer connL.Close()
+	collector := new(manualCollector)
+	clock := &manualClock{current: time.Now()}
+	agent := &manualAgent{}
+	agent.start = func(id [TransactionIDSize]byte, deadline time.Time) error {
+		go agent.h(Event{
+			TransactionID: id,
+			Message:       response,
+		})
+		return nil
+	}
+	c, err := NewClient(ClientOptions{
+		Agent:      agent,
+		Collector:  collector,
+		Connection: connR,
+		Clock:      clock,
+	})
+	if err != nil {
+		t.Fatal(err)
+	}
+	c.SetRTO(time.Second)
+	connClosed := make(chan struct{})
+	go func() {
+		defer func() {
+			connClosed <- struct{}{}
+		}()
+		buf := make([]byte, 1500)
+		for {
+			readN, readErr := connL.Read(buf)
+			if readErr != nil {
+				if readErr == io.EOF {
+					break
+				}
+				t.Error(readErr)
+			}
+			if !IsMessage(buf[:readN]) {
+				t.Error("should be STUN")
+			}
+		}
+	}()
+	wg := new(sync.WaitGroup)
+	for i := 0; i < concurrency; i++ {
+		wg.Add(1)
+		go func() {
+			defer wg.Done()
+			if doErr := c.Do(MustBuild(TransactionID, BindingRequest), func(event Event) {
+				if event.Error != nil {
+					t.Error("failed")
+				}
+			}); doErr != nil {
+				t.Error(doErr)
+			}
+		}()
+	}
+	wg.Wait()
+	if connErr := connR.Close(); connErr != nil {
+		t.Error(connErr)
+	}
+	<-connClosed
+}
 
+func TestClient_DoConcurrent(t *testing.T) {
+	t.Parallel()
+	for _, concurrency := range []int{
+		1, 5, 10, 25, 100, 500,
+	} {
+		t.Run(fmt.Sprintf("%d", concurrency), func(t *testing.T) {
+			testClientDoConcurrent(t, concurrency)
+		})
+	}
 }
diff --git a/go.test.sh b/go.test.sh
index 8c3b012..b4ffaf7 100755
--- a/go.test.sh
+++ b/go.test.sh
@@ -12,6 +12,9 @@ go test ./...
 # test with "debug" tag
 go test -tags debug ./...
 
+# test concurrency
+go test -race -cpu=1,2,4 -run TestClient_DoConcurrent
+
 for d in $(go list ./... | grep -v vendor); do
     go test -race -coverprofile=profile.out -covermode=atomic "$d"
     if [ -f profile.out ]; then