In a community based prevalence study of HIV infection in Bissau, West Africa, 1987, the population in 100 randomly selected "houses" was asked to participate. 89% (1329/1499) were examined and had a blood sample taken. None was HIV-1 seropositive but 4.7% were seropositive for HIV-2 (0.6% in children, 8.9% in those aged 15 years and over, and 20% in those aged 40 years and over). There was no significant difference in seroprevalence between areas or ethnic groups or between individuals of different civil status when age was taken into account. Sexual contact and blood transfusions were the dominant transmission routes, and no case of vertical transmission was identified. The HIV-2 seroprevalence in spouses of HIV-2 seropositive index persons was 40%. For a history of blood transfusion the relative risk of being HIV-2 seropositive was 103.6 in children and 2.4 for adults. After exclusion of spouses, no clustering of HIV-2 seropositivity was seen. At follow-up, after a mean observation time of 325 days, there was an excess mortality for HIV-2 seropositives. The relative risk of dying for HIV-2 seropositive children was 60.8 and for adults 5.0.