OBJECTIVE: To examine the association between autism spectrum disorder (ASD) symptoms and (a) social functioning, (b) mental health, (c) quality of life and (d) sleep in children with and without attention-deficit hyperactivity disorder (ADHD). METHODS: Participants were 6-10-year-old children with ADHD (N=164) and without ADHD (N=198). ADHD was assessed via community-based screening (wave 1) and case confirmation using the Diagnostic Interview Schedule for Children IV (DISC-IV) (wave 2). ASD symptoms were identified using the Social Communication Questionnaire (SCQ). Outcome measures were social functioning (Strengths and Difficulties Questionnaire (SDQ)), mental health (DISC-IV, SDQ), quality of life (QoL, Pediatric Quality of Life Inventory 4.0) and sleep problem severity. RESULTS: Greater ASD symptoms were associated with more parent and teacher-reported peer problems and emotional and conduct problems. For every SD increase in SCQ scores, internalising (OR 1.8, 95% CI 1.3 to 2.6, p=0.001) and externalising disorders (OR 1.5, 95% CI 1.1 to 2.1, p=0.02) increased, QoL decreased by 6.7 units (p<0.001), and moderate/severe sleep problems increased (OR 1.5, 95% CI 1.0 to 2.2, p=0.04). Most findings held in analyses adjusting for socio-demographic factors, ADHD symptom severity and comorbidities (when not the outcome), with the exception of externalising disorders and sleep problems. CONCLUSIONS: ASD symptoms are associated with poorer functioning in children with ADHD. It is important to identify and potentially manage ASD symptoms in children with ADHD given that they exacerbate functional impairments in this already vulnerable group.