The AIDS epidemic in Nigeria is generalized, with infection primarily occurring through heterosexual transmission. It is important to understand patterns of sexual behavior to assess their impact on the epidemic and to design appropriate intervention strategies. This study examined risk factors for extramarital sex among Nigerian men, with a particular focus on polygyny and peri- and postpartum abstinence. Data from the 2003 Nigeria Demographic and Health Survey were analyzed for 1153 men and their wives. Eleven percent of men reported extramarital sex in the previous year. Logistic regression models showed that men with 3 or more wives were at the greatest risk for extramarital sex, followed by monogamous men, when compared with men with 2 wives. Other significant predictors included region, religion, wealth, age at sexual debut, and self-perceived risk of HIV infection. Peri- and postpartum abstinence was not significant. Based on these findings, HIV prevention programs should include men with 3 or more wives and those living in the southwest region, in addition to activities targeting men of all ages. Given the heterogeneity within Nigeria, further in-depth studies should be undertaken to explore the relation between number of wives, peri- and postpartum abstinence, and extramarital sex within specific communities.