Attention deficit hyperactivity disorder (ADHD) is a familial disorder that is highly comorbid with conduct disorder and sometimes co-occurs with bipolar disorder. This pattern of comorbidity is also seen among relatives of ADHD probands. A growing literature suggests that ADHD with antisocial comorbidity may be nosologically distinct from other forms of ADHD. A similar pattern has been observed for ADHD and bipolar disorder. Given these results, along with the observed comorbidity between conduct and bipolar disorders, we used data from our study of 140 ADHD and 120 control families to determine if conduct and bipolar disorders in ADHD boys should be considered alternative manifestations of the same familial disorder. The probands and their relatives were examined with DSM-III-R structured diagnostic interviews and were assessed for cognitive, achievement, social, school, and family functioning. Our results provide fairly consistent support for the hypothesis that antisocial- and bipolar-ADHD subtypes are different manifestations of the same familial condition. As predicted by this hypothesis, there was a significant three-way association between variables assessing the family history of each disorder. Moreover, when families were stratified into bipolar, antisocial, and other types, few differences emerged between the bipolar and antisocial families.