BACKGROUND: Persons with absent partners may be more vulnerable to risky sexual behavior and therefore HIV. Partner absence can be due to traveling (e.g., family visits or funerals) or to living apart (e.g., work-related or in polygamous marriages). We investigated to what extent partner absence leads to more risky sexual behavior in Tanzanian couples. METHODS: We compared 95 men and 85 women living apart with 283 men and 331 women living together. Only persons who were still married were included, either living apart or cohabiting at the time of the interview. Subjects were classified into 4 groups: coresidents being either nonmobile or mobile, and people living apart either frequently or infrequently seeing each other. RESULTS: Most people living apart were polygamously married. Men living apart did not report more extramarital sex than coresident men. However, among coresident men, extramarital sex was reported by 35% of those being mobile compared with 15% of those nonmobile. Among women, those living apart reported extramarital sex more often than coresidents (14% vs. 7%), and this was mainly due to women living apart who infrequently saw their husbands. CONCLUSIONS: Risky sexual behavior occurs more often in mobile coresident men, and in women living apart infrequently seeing their spouses. These groups are relatively easy to identify and need extra attention in HIV prevention campaigns.