The prevalence of human immunodeficiency virus (HIV) infection among adults who are mentally retarded is not known. Policies for those in residential settings are being established despite incomplete information. Knowledge regarding HIV seroprevalence would enable administrators to make more effective policy decisions concerning testing and HIV prevention. Discarded sera from mentally retarded adults were anonymously tested for HIV antibody. Sera were collected from a health facility in Westchester County, NY, that provides care to developmentally disabled adults. After identifications were removed, sera were coded and linked to demographic and clinical variables from hospital and laboratory records. Sera came from individuals living in both institutional and less restrictive community settings in metropolitan New York City and more distant locations in New York State, all of whom were seen by the above facility. No HIV antibody was detected in sera from 241 mentally retarded adults. This study suggests that the prevalence of HIV antibody in mentally retarded adults is not high. Mandatory screening programs may not be appropriate for these individuals. Monies might be better spent on educational programs directed at AIDS prevention, and further development of ethical and safe policies for those who are mentally retarded.