We investigated the relationship between challenging behavior and co-morbid psychopathology in adults with intellectual disability (ID) and autism spectrum disorders (ASDs) (N=124) as compared to adults with ID only (N=562). All participants were first time referrals to specialist mental health services and were living in community settings. Clinical diagnoses were based on ICD-10 criteria and presence of challenging behavior was assessed with the Disability Assessment Schedule (DAS-B). The analyses showed that ASD diagnosis was significantly associated with male gender, younger age and lower level of ID. Challenging behavior was about four times more likely in adults with ASD as compared to non-ASD adults. In those with challenging behavior, there were significant differences in co-morbid psychopathology between ASD and non-ASD adults. However, after controlling for level of ID, gender and age, there was no association between co-morbid psychopathology and presence of challenging behavior. Overall, the results suggest that presence of challenging behavior is independent from co-morbid psychopathology in adults with ID and ASD. (Contains 3 tables.)