Familial risk for bipolar I disorder is associated with erythrocyte omega-3 polyunsaturated fatty acid deficits in youth with attention-deficit hyperactivity disorder

•ADHD and a family history of bipolar I disorder (BD) are risk factors for developing BD in youth.•Developmental omega-3 polyunsaturated fatty acid (n-3 PUFA) deficiency is a putative risk factor for BD.•ADHD youth with a BD family history exhibit a more severe clinical profile, including greater manic symptoms, compared with ADHD youth without a BD family history.•ADHD youth with, but not without, a BD family history also exhibit erythrocyte n-3 PUFA deficits.