The literature relating to the social context of sexual relations in East and Central Africa has several implications for the heterosexual transmission of human immunodeficiency virus (HIV). Colonially created cities in the region still discriminate economically and socially against women. Rapid urbanization is occurring, but migrants maintain strong ties with rural areas. Traditional attitudes towards marriage and sexuality affect urban behavior in the extent of marital stability, the frequency of polygyny, and the emotional bond between spouses. Ethnic groups in Kampala and Nairobi exemplify the cultural foundations of two forms of sexual relations found in the region, one characterized by prostitution and the other by small circles of interchanging lovers. The first results in a more rapid spread of HIV through the urban population and outwards into rural areas. Each pattern exerts unique constraints on behavioral change and requires different prevention campaigns.