The majority of victims of the HIV/AIDS epidemic are found in sub-Saharan Africa. The fact that the African epidemic is almost entirely heterosexual in transmission and that pronounced regional differentials exist in its incidence allows a unique opportunity for studying the mechanisms whereby the epidemic has spread. The ecological and clinical evidence renders it virtually certain that the location of the major AIDS belt extending from Uganda to Zimbabwe has been determined by the lack of male circumcision in this area. This probably enhances the direct transmission of HIV and makes more likely the transmission of genital ulcer diseases, especially chancroid, which act as cofactors for HIV transmission. The epidemic will likely intensify in the areas where males are not circumcised, but elsewhere will largely remain restricted to high-risk groups, such as prostitutes and their regular clients. Beyond Africa, there may be a considerable epidemic risk in the non-Muslim (and hence non-circumcising) areas of Asia. Here much depends on the extent of nonmarital and commercial sexual activity, the level of intravenous drug use, and the extent to which genital ulcerating disease is contained.