Relatively less attention has been paid to reproductive health problems facing deprived urban residents than to those facing rural residents in sub-Saharan Africa. This is probably because the majority of Africans live in rural areas, where they are presumed to have poorer medical, educational, and other social services. Yet, the unprecedented rate of urbanization and the accompanying disproportionate growth in the proportion of poor city residents pose new challenges for health care in the region. This study examines differences in sexual behaviour between slum residents and non-slum residents in Nairobi city. The results show that slum residents start sexual intercourse at earlier ages, have more sexual partners, and are less likely than other city residents to know of or adopt preventive measures against contracting HIV/AIDS. The findings highlight the need to treat slum residents as a subpopulation uniquely vulnerable to reproductive health problems, and to expend more resources in slum settings.