Increasingly, acquired immunodeficiency syndrome (AIDS) prevention programs have been developed to reach and influence street-based populations. Standard methods of evaluation do not fit the conditions of such programs. This article describes a process and outcome evaluation of an AIDS prevention program for sex workers in which qualitative and quantitative methods were combined in order to mediate research problems endemic to street-based populations. Methods included epidemiological questionnaires, open-ended interviews with participants, and ethnographic field notes. Process evaluation findings show that field staff who were indigenous to the neighborhood and population readily gained access to the community of sex workers and simultaneously became role models for positive behavior change. Outcome findings show that sex workers do feel at risk for AIDS, but usually from clients rather than from husbands or boyfriends. Accordingly, they use condoms more frequently with clients than with steady partners. Increasing condom use among sex workers with their steady partners remains an important challenge for AIDS prevention. Combining qualitative and quantitative research data provided a more comprehensive assessment of how to reach sex workers with effective AIDS risk reduction messages than either method could have provided alone.