Background: No consensus has been reached yet on how best to characterize children with juvenile bipolar disorder (JBD). Several groups have shown that children on the attention problems (AP), aggressive behavior (AGG), and anxious-depressed (AD) syndromes of the Child Behavior Checklist (CBCL) are likely to meet criteria for DSM-JBD. We aimed to use a large population-based twin sample to evaluate the prevalence and genetic architecture of the CBCL-JBD (deviant on AP, AGG, and AD) phenotype and compare these data to children who are deviant on just the CBCL-AP syndrome. Methods: Structural equation modeling (SEM) was applied to CBCL data from 5418, 3562, and 1971 Dutch twin pairs at ages 7, 10, and 12 years. Results: The CBCL-JBD phenotype occurs in ∼1% of children at each age. Among the children who meet criteria for the CBCL-AP phenotype (∼ 5%), between 13 and 20% also meet criteria for CBCL-JBD. The best SEM for CBCL-JBD includes additive genetic, shared and unique environmental factors. The best SEM for CBCL-AP includes dominant and additive genetic and unique environmental factors. Conclusions: These data suggest that CBCL-JBD is common, and even more common among children who have severe attention problems. CBCL-JBD shows familial aggregation due to both genetic and shared environmental factors. © 2005 Society of Biological Psychiatry.