OBJECTIVE: The relationship between attention deficit hyperactivity disorder (ADHD) and earlier age at onset of affective illness was examined in probands with a history of bipolar disorder. METHOD: The authors assessed 56 adult bipolar subjects. Those with a history of childhood ADHD (N=8) were age and sex matched with bipolar subjects without a history of childhood ADHD (N=8). RESULTS: The age at onset of the first affective episode was lower for the subjects with bipolar disorder and a history of childhood ADHD (mean=12.1 years, SD=4.6) than for those without a history of childhood ADHD (mean=20.0 years, SD=11.3). CONCLUSIONS: ADHD in children of bipolar probands might identify children at highest risk for development of bipolar disorder.