The seroprevalence of human immunodeficiency virus (HIV) infection in Guinea-Bissau, West Africa, was determined by enzyme-linked immunosorbent assay (ELISA). From January 1987 to February 1993, 590 patients from the outpatient and inpatient departments of Regional Hospital at Canchungo, Cacheu, Guinea-Bissau were studied. The overall seropositive rate was 16%. Patients in the age between 25 and 54 accounted for 78% of HIV-positive cases. The seropositive rate according to the diagnosis was: 6% in pregnant women, 40% in patients with gonorrhea/syphilis, 14% in patients with vaginitis and 22% in patients with active pulmonary tuberculosis. The seropositivity for HIV-1/2 in the pregnant women might reflect the seroprevalence in general population of Guinea-Bissau. Accordingly, the estimated population infected by HIV would be sixty thousands in Guinea-Bissau. Both sexually-transmitted diseases and tuberculosis were the risk factors for HIV infection. This study shows that HIV infection is a critical problem of public health in Guinea-Bissau. Strategies to prevent the seeding of HIV are of great importance. Moreover, the members of medical mission from our country must keep alert for preventing HIV infection.