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.