CONTEXT: Since 1990, HIV infection in Brazil has spread among the heterosexual population, particularly in the north. Containment of the epidemic can be informed by a better understanding of men's sexual risk behavior. METHODS: Logistic, Poisson and multilevel logit models were applied to data on married and cohabiting men who had participated in the 1996 Brazilian Demographic and Health Survey. RESULTS: Twelve percent of married or cohabiting men reported having had at least one extramarital partner in the previous 12 months; half of these had had two or more. The majority (77%) of partners were described as friends or lovers; 4% had been prostitutes and 15% strangers. Among men who had had sex with an extramarital partner in the last year, 40% reported having used condoms during last extramarital sex. Compared with members of evangelical religions, other men were significantly more likely to report having had an extramarital partner (odds ratios, 3.0-4.7) and unprotected extramarital sex in the last 12 months (3.4-7.9). Region of residence was also strongly correlated with extramarital sex: Compared with men in southern or central Brazil, those in the north had more than three times the odds of having had extramarital sex and unprotected extramarital sex in the last year (3.1-3.8). CONCLUSION: In Brazil, religious affiliation and region of residence exert a major influence on risk behavior.