Examined predictors of substance use disorders in nonreferred siblings of boys with and without attention deficit hyperactivity disorder to further investigate whether previous findings documenting the role of social impairment in predicting substance use disorders would be replicated. Participants were comprehensively assessed at Time 1 and at 4-year follow-up. We found that social impairment was the sole significant predictor of alcohol and substance abuse and smoking after controlling for other variables previously shown to be predictors of substance use disorders. These results confirmed prior findings documenting the critical role of social impairment in predicting later substance use disorders.