AbstractThe prevalence of dual diagnosis (developmental handicap with accompanying mental illness) was examined in a population of 71 persons who are institutionalized and developmentally handicapped. The Reiss Screen for Maladaptive Behavior (Reiss, 1988) revealed that 69% of the popula‐ tion had a dual diagnosis. The incidence of dual diagnosis was not related to level of functioning, although the type of psychopathology varied considerably across levels of functioning. In contrast to findings from the Reiss Screen, previous psychiatric diagnosis indicated a much lower rate of dual diagnosis (26.8%), as did current psychiatric assessment. There was little agreement between the two psychiatric assessments, or between the Reiss Screen and the psychiatric assessments. The Reiss Screen appears to provide information of more practical value for community placement decisions than is provided by the psychiatric assessments. Limitations to the concept of ‚dual diagnosis’︁ for characterizing the mental health needs of persons with a developmental handicap are di