This study reports the results of research designed to determine the prevalence of HIV infection in a group of male clients of brothel prostitutes, and to describe characteristics associated with HIV infection in Dakar, Senegal. Clients come from the lower portion of the social scale rather than from the middle class or from the wealthier groups of the population. A significant number of these men did not use condoms with their regular partners, and another group reported sexual contact with occasional partners with whom condom use was not usual. Additionally, for a fraction, condoms were not systematically used either with prostitutes or with partners. HIV prevalence in clients appeared to be much higher than the prevalence in the general population and HIV-infected clients were older than HIV-negative clients. The significant association between HIV infection and age can be explained by the fact that older clients probably have been exposed to prostitution longer. HIV-positive clients had more contact with prostitutes during the previous seven days, and in addition they also had more occasional sexual encounters than did the HIV-negative clients. This demonstrates that a multiplicity of sexual partners increases the risk of HIV infection beyond the contact with prostitutes for this group of men.