Professional social workers are called on to assist many vulnerable clientpopulations. One such group consists of adults with coexisting substance use and mental disorders who are at risk for HIV infection. This article presents an evaluation of a social group work intervention designed to help dually disordered adults change sexual behaviors that increase their risk of HIV infection. Thirty clients attending a day treatment program for dually disordered adults agreed to participate in an AIDS awareness and prevention group. Using a nonequivalent control group evaluation design, clients were systematically assigned to either the first cycle of the group or to a waiting list. Findings demonstrated that, compared to persons on the waiting list, clients who received the preventive intervention improved their understanding of correct condom use, used condoms more often, reduced risky sexual practices, and were more assertive in sexual encounters. Behavioral changes were attributable to group involvement and not the clients' pregroup characteristics. This was a limited pilot evaluation. The results, however, should encourage social workers developing AIDS preventive interventions for a particularly vulnerable client population.