Differential diagnosis in client populations presenting with complexities of developmental disabilities has become a time honored psychodiagnostic practice. Standard practice has been to ignore comorbidity in client groups where mental disorders and retardation coexist. In persons with mental retardation, differential diagnosis prioritizes the many presenting symptoms, identifying a single causation. Attention deficit hyperactivity disorders, mood disorders, pervasive developmental disorders, stereotypic movement disorders, and mental disorders due to a general medical condition often coexist with mental retardation. As mentally retarded populations move from institutional to community residency, comorbidity appears more prevalent, often becoming a client management issue. Dual diagnosis is important in considering treatment plans, obtaining access and funding for services. interdisciplinary communication, and generating epidemiological data.