The authors examined the siblings of 140 attention-deficit hyperactivity disorder (ADHD) and 120 control probands and classified families as antisocial if the proband had conduct disorder or a parent had antisocial personality. Partial support was found for the hypothesis that the ADHD gender effect would be limited to antisocial families. Boys had an increased risk for ADHD compared with girls, but only among siblings from antisocial families. The effect size for predicting ADHD in siblings of probands was greater for maternal compared with paternal ADHD, but only for families exhibiting antisocial disorders. Strong support was found for the hypothesis that, compared with siblings from nonantisocial families, those from antisocial families would have more psychopathology (ADHD, depression, substance use, and conduct disorders). The presence of antisocial disorders signals a distinct subtype of ADHD.