Many STD/HIV-prevention programs worldwide assume that individuals' risk of acquiring sexually transmitted diseases, including HIV infection, is highest in the context of commercial sex. To address this assumption, research conducted in urban Southwest Nigeria combined qualitative and quantitative methods to examine men's sexual behavior, condom use, and STD experience in different types of sexual relationships (marital, casual, and commercial). Logistic regression analysis of survey data indicates that number of sexual partners and sex with sex workers are positively and significantly related to STD experience. Follow-up in-depth interviews with clients of sex workers indicate, however, that these men are actually more likely to report having contracted an STD from a casual sex partner than from a sex worker. Men are most uncertain about their vulnerability to STDs with casual partners. Men's condom use is highest in commercial sex, inconsistent in casual relationships, and lowest in marriage. STD/HIV-prevention programs need to address the range of sexual relationships and the meanings and behaviors associated with them.