The interrelationship between the Autism Diagnostic Interview-Revised (ADI-R), Autism Diagnostic Observation Schedule-Generic (ADOS-G) and clinical classification was studied in 184 children and adolescents with Mental Retardation (MR). The agreement between the ADI-R and ADOS-G was fair, with a substantial difference between younger and older children (5-8 vs. 8+ years). Compared with the Diagnostic and Statistical Manual of Mental Disorders-IV-TR (DSM-IV-TR) classification of Autistic Disorder (AD) and Pervasive Developmental Disorder (PDD), both instruments measure AD or PDD validly and reliably. Even in low-functioning children the interrelationship between the instruments and the clinical classification was satisfactory. The combination of ADI-R and ADOS-G identifies AD or PDD, as described in the DSM-IV-TR, most appropriately. Both instruments seem to be of great value in the diagnostic process of PDD in children and adolescents with MR.