OBJECTIVE: To assess the implications of autistic traits (ATs) in youth with attention-deficit/hyperactivity disorder (ADHD) without a diagnosis of autism. METHODS: Participants were youth with (n = 242) and without (n = 227) ADHD and controls without ADHD in whom a diagnosis of autism was exclusionary. Assessment included measures of psychiatric, psychosocial, educational, and cognitive functioning. ATs were operationalized by using the withdrawn + social + thought problems T scores from the Child Behavior Checklist. RESULTS: A positive AT profile was significantly overrepresented among ADHD children versus controls (18% vs 0.87%; P < .001). ADHD children with the AT profile were significantly more impaired than control subjects in psychopathology, interpersonal, school, family, and cognitive domains. CONCLUSIONS: A substantial minority of ADHD children manifests ATs, and those exhibiting ATs have greater severity of illness and dysfunction.