diff --git a/appender.go b/appender.go
index be8007d..0d9a1b6 100644
--- a/appender.go
+++ b/appender.go
@@ -240,6 +240,7 @@ func (a *Appender) flush(ctx context.Context, bulkIndexer *bulkIndexer) error {
 	logger := a.config.Logger
 	if a.tracingEnabled() {
 		tx := a.config.Tracer.StartTransaction("docappender.flush", "output")
+		tx.Context.SetLabel("documents", n)
 		defer tx.End()
 		ctx = apm.ContextWithTransaction(ctx, tx)
 
diff --git a/appender_test.go b/appender_test.go
index 525daaa..2c733d7 100644
--- a/appender_test.go
+++ b/appender_test.go
@@ -32,6 +32,7 @@ import (
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
 	"go.elastic.co/apm/v2/apmtest"
+	"go.elastic.co/apm/v2/model"
 	"go.uber.org/zap"
 	"go.uber.org/zap/zapcore"
 	"go.uber.org/zap/zaptest/observer"
@@ -876,6 +877,9 @@ func testAppenderTracing(t *testing.T, statusCode int, expectedOutcome string) {
 
 	assert.Equal(t, expectedOutcome, payloads.Transactions[0].Outcome)
 	assert.Equal(t, "output", payloads.Transactions[0].Type)
+	assert.Equal(t, model.IfaceMapItem{Key: "documents", Value: float64(N)},
+		payloads.Transactions[0].Context.Tags[0],
+	)
 	assert.Equal(t, "docappender.flush", payloads.Transactions[0].Name)
 	assert.Equal(t, "Elasticsearch: POST _bulk", payloads.Spans[0].Name)
 	assert.Equal(t, "db", payloads.Spans[0].Type)