Psychiatric disorders in individuals with co-occurring autism spectrum disorders (ASD) and intellectual disability (ID) are common, but diagnosis presents many challenges. The Psychopathology in Autism checklist (PAC) is among the very few instruments specifically developed for this group of individuals. The psychometric properties of the PAC (i.e. criterion validity, specificity, sensitivity and predictive values) were explored by comparing scores with assessments on the Aberrant Behavior Checklist (ABC) and examining how well assessment by the PAC at referral predicts final clinical diagnoses. Results indicated a significant correlation with the ABC, further supporting the validity of the PAC. Sensitivity and Specificity for specific diagnoses were variable, although positive predictive value for "any diagnosis' was relatively high. The study confirms the potential value of the PAC as a screening checklist but highlights the need for clinical diagnosis to be based on a multimodal, multidisciplinary assessment.