Psychologists interviewed direct-care staff using a battery of assessment measures including the autism spectrum disorders-diagnosis for intellectually disabled adults (ASD-DA), the Diagnostic Assessment for the Severely Handicapped-II (DASH-II), the Matson Evaluation of Social Skills for Individuals with Severe Retardation (MESSIER), the Socialization domain of the Vineland Adaptive Behavior Scales (VABS), and a checklist containing criteria for autism and PDD-NOS from the DSM-IV-TR and ICD-10. Three hundred and seven intellectually disabled (ID) adolescents and adults ranging in age from 16 to 88 were assessed. Participants were diagnosed with either ID and ASD (autism or PDD-NOS; n = 156) or ID and no Axis I diagnosis (n = 151). A modification of the multitrait-multimethod approach was used to establish the convergent and discriminant validity of the ASD-DA. The scale proved to have robust convergent validity when correlated with the DSM-IV-TR/ICD-10 checklist, MESSIER, and Socialization domain of the VABS. Additionally, discriminant validity was demonstrated by comparing the ASD-DA to items from the DASH-II (measure of general psychopathology). The implications of these data are discussed. (Contains 3 tables.)