diff --git a/tools/src/cts/expectations/validate.go b/tools/src/cts/expectations/validate.go
index 6ff805d6b4..8571d553d4 100644
--- a/tools/src/cts/expectations/validate.go
+++ b/tools/src/cts/expectations/validate.go
@@ -51,6 +51,15 @@ func (c Content) Validate() Diagnostics {
 	var out Diagnostics
 	for _, chunk := range c.Chunks {
 		for _, ex := range chunk.Expectations {
+			for _, status := range ex.Status {
+				if status == "Slow" {
+					out = append(out, Diagnostic{
+						Severity: Error,
+						Line:     ex.Line,
+						Message:  fmt.Sprintf("\"Slow\" expectation is not valid here. Use slow_tests.txt instead."),
+					})
+				}
+			}
 			glob, err := tree.Glob(query.Parse(ex.Query))
 			if err != nil {
 				out = append(out, Diagnostic{
diff --git a/tools/src/cts/expectations/validate_test.go b/tools/src/cts/expectations/validate_test.go
index d9b15b9ff0..55234e1031 100644
--- a/tools/src/cts/expectations/validate_test.go
+++ b/tools/src/cts/expectations/validate_test.go
@@ -116,6 +116,19 @@ crbug.com/a/123 [ os-a ] a:b,c:d,* [ Failure ]
 				},
 			},
 		},
+		{ //////////////////////////////////////////////////////////////////////
+			name: "slow invalid",
+			expectations: `
+crbug.com/a/123 a:b,c:d,* [ Slow ]
+`,
+			diagnostics: expectations.Diagnostics{
+				{
+					Line:     8,
+					Severity: expectations.Error,
+					Message:  "\"Slow\" expectation is not valid here. Use slow_tests.txt instead.",
+				},
+			},
+		},
 	} {
 		ex, err := expectations.Parse("expectations.txt", header+test.expectations)
 		if err != nil {