Policy Research Division Working Paper no. 216. The causes of wide variation in the sizes of HIV epidemics among countries in sub-Saharan Africa are not well understood. This study assesses the potential roles of late age at marriage and a long period of premarital sexual activity as population risk factors for HIV infection in the region. The relationship between marital status and the prevalence and incidence of HIV is examined with ecological data from 33 sub-Saharan African countries and with individual-level data from nationally representative Demographic and Health Surveys in Kenya and Ghana in 2003. The ecological analysis finds a significant positive correlation between HIV prevalence and the median age at first marriage, and between HIV prevalence and the interval between first sex and first marriage. In the individual-level analysis, the risk for HIV infection per year of exposure among sexually active women is higher before than after first marriage. These findings support the hypothesis that a high average age at marriage in a population leads to a long period of premarital sex during which partner changes are relatively common, thus facilitating the spread of HIV.