In sub-Saharan Africa, the prevalence of HIV infection among young women is much higher than that among young men. Many women enter marriage HIV-infected, suggesting that men may be predominantly infected by their wives. Using data from cross-sectional surveys in Kisumu, Kenya, and Ndola, Zambia, in 1997, the prevalence of HIV infection at marriage was estimated from age at marriage and age- and sex-specific prevalence of HIV infection among unmarried individuals. Using a deterministic model, this prevalence was compared with measured concordance of HIV infection among recently married couples to estimate transmission probabilities within marriage and extramarital incidence of HIV infection. Over a wide range of assumptions, we estimated that at least one quarter of cases of HIV infection in recently married men were acquired from extramarital partnerships, and for both men and women, less than one half of cases of HIV infection were acquired from their spouse. In these sites, many infections in married men, even in those with HIV-infected wives, may be acquired from outside the marriage.