OBJECTIVE To determine the prevalence of HIV infection and its relationship to age, sex and other factors. METHODS Cross-sectional survey of a rural community in Guinea-Bissau. METHODS Questionnaire-administration and screening of sera from subjects aged > or = 15 years. RESULTS Of the 2770 subjects tested, 220 (7.9%) were HIV-2-seropositive, four (0.1%) were HIV-1-seropositive and 10 (0.4%) were dually reactive. Overall prevalence of HIV-2 was 9.3% in women, peaking at 17.2% in the 35-44 age group, and 6.6% in men, peaking at 19.1% in the 45-54 age group. The mean age of the four subjects with HIV-1 infection was 24 years, which was significantly lower than those with HIV-2 infection. HIV-2 infection was more prevalent among women who were widowed or divorced, women whose husbands were living away from the study area, and women who had lived in the capital, Bissau. The majority of subjects with an infected spouse remained uninfected and none of the women aged < 25 years whose husbands were infected were seropositive. The prevalence varied significantly between settlements within the study area. CONCLUSIONS The pattern of HIV-2 infection in this rural community has similarities to that found in urban Bissau, and prevalence in both areas peaks in older subjects than in HIV-1 foci. The findings support previous suggestions that HIV-2 is not a recent introduction to Guinea-Bissau, and that it is less pathogenic and less readily transmitted than HIV-1.