AbstractBackgroundMany children with an intellectual or developmental disability (IDD) have associated autism spectrum disorders (ASD), as well as an increased risk of mental health difficulties. In a cohort with IDD of genetic aetiology, we tested the hypothesis that excess risk attached to those with ASD + IDD, in terms of both children's mental health and parental psychological distress.MethodsParticipants with a copy number variant or single nucleotide variant (5–19 years) were recruited via UK National Health Service. 1904 caregivers competed an online assessment of child mental health and reported on their own psychological wellbeing. We used regression to examine the association between IDD with and without co‐occurring ASD, and co‐occurring mental health difficulties, as well as with parental psychological distress. We adjusted for children's sex, developmental level, physical health, and socio‐economic deprivation.ResultsOf the 1904 participants with IDD, 701 (36.8%) had co‐occurring ASD. Children with both IDD and ASD were at higher risk of associated disorders than those with IDD alone (ADHD: OR = 1.84, 95% confidence interval [CI] 1.46–2.32,p< 0.0001; emotional disorders: OR = 1.85, 95%CI 1.36–2.5,p< 0.0001; disruptive behaviour disorders: OR = 1.79, 95%CI 1.36–2.37,p< 0.0001). The severity of associated symptoms was also greater in those with ASD (hyperactivity:B= 0.25, 95%CI 0.07–0.34,p= 0.006; emotional difficulties:B= 0.91, 95%CI 0.67 to 1.14,p< 0.0001; conduct problems:B= 0.25, 95%CI 0.05 to 0.46,p= 0.013). Parents of children with IDD and ASD also reported greater psychological distress than those with IDD alone (β= 0.1, 95% CI 0.85 to 2.21,p< 0.0001). Specifically, in those with ASD, symptoms of hyperactivity (β= 0.13, 95% CI 0.29–0.63,p< 0.0001), emotional difficulties (β= 0.15, 95% CI 0.26–0.51,p< 0.0001) and conduct difficulties (β= 0.07, 95% CI 0.07–0.37,p< 0.004) all significantly contributed to parental psychological distress.ConclusionsAmong children with IDD of genetic aetiology, one third have co‐occurring ASD. Not only do those with co‐occurring ASD present with a wider range of associated mental health disorders and more severe mental health difficulties than those with IDD alone, but their parents also experience more psychological distress. Our findings suggest that the additional mental health and behavioural symptoms in those with ASD contributed to the degree of parental psychological distress.