In order to further characterize the pathogenesis ofToxoplasma gondiiinfection in patients with AIDS and AIDS-related complex (ARC), a cohort of HIV- and Toxoplasma-infected individuals were identified and prospectively followed. Four hundred and 10 HIV-infected individuals followed in the San Francisco General Hospital AIDS Clinic were screened for antibodies to Toxoplasma between November 1986 and November 1988. Of the 67 (16%) individuals seropositive for Toxoplasma antibodies, 33 (49%) were followed monthly for a mean duration of 7.5 months. One hundred and 11 follow-up blood samples were obtained in order to determine Toxoplasma serology and the incidence of parasitemia. In general, Toxoplasma immunoglobulin (lg) C antibodies remained stable over time. Detection of Toxoplasma antigenemia and parasitemia was uniformly negative, including those specimens obtained from two individuals within 45 days of their developing toxoplasmic encephalitis.