In the USA and Europe, toxoplasmosis is well recognized as an important cause of morbidity and mortality among immunocompromised individuals. Toxoplasma gondii has been shown to be a common opportunistic infection in patients infected with the human immunodeficiency virus (HIV) in the USA and Europe with published estimates ranging from 20% to 80%. The importance of Toxoplasma infection in East Africa has not yet been defined. The seroprevalence rates of toxoplasmosis in Zambian and Ugandan patients were determined using the dye test (DT) and the latex agglutination test (LAT). The geographical variation in seroprevalence rates noted in western countries was also found in these African countries, with Zambia showing significantly lower rates than Uganda. 34% of Ugandan (64/186) and 4% of Zambian (8/187) patients infected with HIV, compared with 27% of Ugandan (26/93) and 11% of Zambian (20/189) HIV-negative persons, had anti-Toxoplasma immunoglobulin G antibodies. With the LAT, 13% of the Ugandan and 7% of the Zambian sera gave a false positive result. The relevance of Toxoplasma serology in Africa is discussed.