BACKGROUND: Since little is known as to whether sex differences affect the clinical presentation of pediatric BP-I disorder, it is an area of high clinical, scientific and public health relevance. METHODS: Subjects are 239 BP-I probands (65 female probands, 174 male probands) and their 726 first-degree relatives, and 136 non-bipolar, non-ADHD control probands (37 female probands, 99 male probands) and their 411 first-degree relatives matched for age and sex. We modeled the psychiatric and cognitive outcomes as a function of BP-I status, sex, and the BP-I status-gender interaction. RESULTS: BP-I disorder was equally familial in both sexes. With the exception of duration of mania (shorter in females) and number of depressive episodes (more in females), there were no other meaningful differences between the sexes in clinical correlates of BP-I disorder. With the exception of a significant sex effect for panic disorder and a trend for substance use disorders (p=0.05) with female probands being at a higher risk than male probands, patterns of comorbidity were similar between the sexes. Despite the similarities, boys with BP-I disorder received more intensive and costly academic services than girls with the same disorder. LIMITATIONS: Since we studied children referred to a family study of bipolar disorder, our findings may not generalize to clinic settings. CONCLUSIONS: We found more similarities than differences between the sexes in the personal and familial correlates of BP-I disorder. Clinicians should consider bipolar disorder in the differential diagnosis of both boys and girls afflicted with symptoms suggestive of this disorder.