Despite ongoing concerns that traumatized children with severe symptoms of emotional dysregulation may be inappropriately receiving a diagnosis of pediatric bipolar-I (BP-I) disorder, this issue has not been adequately examined in the literature. Because both pediatric BP-I disorder and PTSD are familial disorders, if children with both BP-I and PTSD were to be truly affected with BP-I disorder, their relatives would be at high risk for BP-I disorder. To this end, we compared patterns of familial aggregation of BP-I disorder in BP-I children with and without PTSD with age and sex matched controls. Participants were 236 youth with BP-I disorder and 136 controls of both sexes along with their siblings. Participants completed a large battery of measures designed to assess psychiatric disorders, psychosocial, educational, and cognitive parameters. Familial risk analysis revealed that relatives of BP-I probands with and without PTSD had similarly elevated rates of BP-I disorder that significantly differed from those of relatives of controls. Pediatric BP-I disorder is similarly highly familial in probands with and without PTSD indicating that their co-occurrence is not due to diagnostic error.