diff --git a/test/e2e/horizontal_pod_autoscaling.go b/test/e2e/horizontal_pod_autoscaling.go
index 09d8467f783..a64bb638700 100644
--- a/test/e2e/horizontal_pod_autoscaling.go
+++ b/test/e2e/horizontal_pod_autoscaling.go
@@ -31,8 +31,10 @@ const (
 	subresource    = "scale"
 )
 
+// These tests don't seem to be running properly in parallel: issue: #20338.
+//
 // These tests take ~20 minutes each.
-var _ = Describe("Horizontal pod autoscaling (scale resource: CPU) [Slow]", func() {
+var _ = Describe("Horizontal pod autoscaling (scale resource: CPU) [Serial] [Slow]", func() {
 	var rc *ResourceConsumer
 	f := NewFramework("horizontal-pod-autoscaling")