BACKGROUND Mental disorder and intellectual disability each accounts for substantial burden of disease. However, the extent of this co-occurrence varies substantially between reports. We sought to determine whether studies in children and/or adolescents with acceptably rigorous methods can be distinguished from existing reports, and whether key risk factors could be ascertained. METHODS Published studies investigating the prevalence of mental disorders in children and/or adolescents with intellectual disability were reviewed. RESULTS Nine studies with acceptable methods were identified, 4 which compared the prevalence of mental disorder in populations of those with and without intellectual disability, and a further 5 studies that estimated the rates of mental disorder in those with intellectual disability were identified. Collectively, these studies demonstrate rates of comorbidity for children and adolescents between 30 and 50% with a relative risk of mental disorder associated with intellectual disability ranging from 2.8-4.5. The risks for this comorbidity associated with age, gender, severity of intellectual disability, and socioeconomic status remain uncertain. CONCLUSIONS Appreciation of this comorbidity needs to be a fundamental component of both mental health and intellectual disability services.