Psychiatric disorders are common in youth with intellectual and developmental disabilities. This is a vulnerable group of children whose behavioural problems often have more complicated care needs than other children, which can place a great deal of stress on their families. However, the association of family mental health issues, level of intellectual ability, and diagnostic co-morbidity in children is relatively under-studied. In the present study, we investigated the relationship among child diagnoses, family mental health problems, risk for self-injury, and disruption in care among children with (N = 517) and without (N = 517) intellectual and developmental disabilities. A negative binomial regression showed that mental health problems in multiple family members, self-injurious behaviour, and self-reported abuse/trauma was related to greater likelihood of provisional diagnoses of co-occurring psychiatric disorders in both a clinically referred sample and a sample with IDD. Implications for care-planning are discussed.