with government mandates to deinstitutionalize it is important to investigate the prevalence of dual diagnosis (developmental handicap with accompanying mental illness) in institutional settings for persons with a developmental handicap. assessment for mental illness of 71 institutionalized developmentally handicapped adults was done using the reiss screen for maladaptive behavior (reiss, 1988). prevalence of dual diagnosis, identified by the reiss screen, was 69%. the most frequent categories of dual diagnosis were aggression disorder. sexual problems. self-injurious behaviour, and stealing. the reiss screen identified a much higher prevalence of dual diagnosis than either previous or current psychiatric diagnosis. there was a high level of dual diagnosis regardless of level of functioning; however, the type of pathology varied considerably across levels of functioning. despite lack of validation of the reiss screen by previous and current psychiatric diagnosis, the reiss screen may in fact be a useful and valid instrument for measuring those maladaptive behaviours that prevent dually diagnosed and other developmentally handicapped persons from successful community placement. the high prevalence of persons with these behaviours has implications for planning and implementing support services in the community.