During a three-week period in March/April 1987, the authors examined 253 consecutive patients referred to a rural hospital in northwestern Tanzania. Sera were tested for antibodies to human immunodeficiency virus type 1 (HIV-1), human immunodeficiency virus type 2 (HIV-2), and human T-lymphotropic virus type I (HTLV-I), as well as for various parasites, hepatitis B virus, and Treponema pallidum. Neopterin (urinary and serum) was chosen as the immunologic parameter. In eight of the 253 patients (3.2%), a clinical diagnosis of acquired immunodeficiency syndrome (AIDS) was established. Three of the AIDS patients had HIV-1 antibodies, two had HIV-1 antigen, one had both HIV-1 and HIV-2 antibodies, and in one patient, only HIV-2 antibodies were found. The total HIV-1 and HIV-2 seroprevalence (antibodies plus antigen) was 4.3%; HTLV-I seroprevalence was 9.9%. No correlation could be found between HIV (or HTLV-I) seropositivity and raised levels of antibody to the above pathogens. There was, however, a significantly positive correlation between HIV seropositivity and history of gonorrhea, whereas a history of operations, injections, vaccinations, blood transfusions, or scarification did not influence the level of HIV seropositivity. The most frequently noted epidemiologic association with HIV seropositivity was traveling to or coming from Uganda or Rwanda. Two thirds of the studied Tanzanians had elevated neopterin levels, and all seven HIV-seropositive patients with clinical signs of AIDS had extremely high serum and urinary neopterin levels compared with HIV-seropositive patients without signs of AIDS. Increased neopterin levels reflect a stimulation of the T-cell/macrophage system.