diff --git a/src/Microsoft.ML.Data/Prediction/PredictionEngine.cs b/src/Microsoft.ML.Data/Prediction/PredictionEngine.cs
index 43d71555ba..c1639958bd 100644
--- a/src/Microsoft.ML.Data/Prediction/PredictionEngine.cs
+++ b/src/Microsoft.ML.Data/Prediction/PredictionEngine.cs
@@ -62,6 +62,8 @@ public void Reset()
///
/// This class can also be used with trained pipelines that do not end with a predictor: in this case, the
/// 'prediction' will be just the outcome of all the transformations.
+ ///
+ /// The PredictionEngine is NOT thread safe. Using it in a threaded environment can cause unexpected issues.
///
public sealed class PredictionEngine : PredictionEngineBase
where TSrc : class