HTLV-1 infections are particularly prevalent in parts of Japan, the Caribbean, and West Africa, with the virus apparently endemic in Africa for centuries with only a minor impact upon the population's health. Findings are reported from a study conducted to assess the impact of HTLV-1 infection upon overall survival and to investigate the risk factors for HTLV-1. In 1989, a population-based cohort of subjects at least 50 years old was established in an urban area of Guinea-Bissau. Overall, 346 people were interviewed in detail about their risk behaviors and had capillary blood drawn. 4.6% of men and 12.4% of women were HTLV-1 seropositive. No case of HTLV-2 was identified. Seropositivity varied considerably according to place of birth and ethnic group. Among women, but not men, HTLV-1 seropositivity was strongly associated with early sexual debut, the lifetime number of male sex partners, and the male partner's number of co-wives. There was also a 3.1-fold increased risk of being HTLV-1 seropositive if the woman was also HIV-2 seropositive. Multivariate analysis found the strongest association with HTLV-1 infection to be an history of having been bitten by a monkey. Ornamental scarification was associated with a 3.3-fold increased risk. Ethnic affiliation also significantly influenced the risk of being HTLV-1 seropositive. Follow-up performed in January 1996 found no difference in survival between HTLV-1-seropositive and -seronegative individuals over the course of 6 years. This population is highly endemic for both HTLV-1 and HIV-2 infection, with sex behaviors clearly being important in the spread of HTLV-1 among women, but nonsexual risk factors only of potential importance among men.