The current study explored the longitudinal relation between parental expressed emotion, a well-established predictor of symptom relapse in various other disorders (e.g., schizophrenia) with externalizing behaviors in 84 children, ages 8-18 (at Time 2), with autism spectrum disorder (ASD). It was found that parental expressed emotion, specifically criticism/hostility at Time 1, significantly related to a change in externalizing behaviors from Time 1 to Time 2, even after controlling for Time 1 family income, ASD symptom severity, parental distress, and parenting practices. That is, higher levels of parental criticism/hostility at Time 1 predicted higher levels of child externalizing behaviors at Time 2. However, the reverse was not found. This finding of a unidirectional relation has important clinical implications.