It is well known that levels of HIV prevalence tend to be appreciably higher inurban areas. This article considers the reasons for this and shows that within world regions that are relatively homogeneous with respect to their experience of HIV/AIDS, variation in the level of urbanization corresponds to about one-third of variation in estimated HIV prevalence. Furthermore, for populations in the world's worst-affected area—eastern and southern Africa—there are signs that, partly by differentially raising urban death rates and depressing urban birth rates, HIV/AIDS is slowing the pace of urbanization. Finally, in countries with very high levels of HIV infection and relatively low birth rates, such as in South Africa, the urban sector will soon constitute a “demographic sink”—with death rates exceeding birth rates.