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To evaluate the effectiveness of pyrimethamine (given with leucovorin calcium versus placebo (an inactive substance) for the primary prophylaxis (prevention) of cerebral toxoplasmosis in HIV-infected patients. Cerebral toxoplasmosis is one of the most frequently encountered opportunistic infections in the course of AIDS. The mortality (death) rate is estimated to be greater than 50 percent. Pyrimethamine is a drug that appears promising for the primary prevention of cerebral toxoplasmosis in HIV-infected patients. Cerebral toxoplasmosis is one of the most frequently encountered opportunistic infections in the course of AIDS. The mortality (death) rate is estimated to be greater than 50 percent. Pyrimethamine is a drug that appears promising for the primary prevention of cerebral toxoplasmosis in HIV-infected patients. AMENDED: 04-04-91 On the first day of therapy, a loading dose is given. After the first day, patients take pyrimethamine or placebo 3 times a week. Patients also take leucovorin calcium orally three times weekly. Enrollment occurs over approximately 12 months. All patients are followed on study until a common study close-out date and final analysis of the study. It is anticipated that this common close-out will occur when the mean duration of time on study therapy will be 3 years (approximately in January, 1994). ORIGINAL design: On the first day of treatment, patients receive a loading dose of pyrimethamine or placebo, plus of leucovorin calcium. After the first day, patients take pyrimethamine or placebo three times a week. Patients also take folinic acid orally three times weekly. The mean duration of study participation is 3 years.
Inclusion Criteria Concurrent Medication: Allowed: * Zidovudine (AZT), didanosine (ddI), dideoxycytidine (ddC), erythropoietin (Eprex), other agents granted Treatment IND or expanded access status. * Investigational triazoles. * Pentamidine for primary prophylaxis of Pneumocystis carinii pneumonia (PCP). Patients with the following are excluded: * History of cerebral toxoplasmosis or toxoplasmosis infection in any other organ or tissue. * Focal neural abnormalities (except peripheral neuropathy) or mass lesions on a previous computerized tomography (CT) scan or magnetic resonance image (MRI), unless subsequent workup rules out toxoplasmosis, in which case abnormalities must have been stable for at least 2 months. * Known or suspected allergy or severe intolerance to study drugs. Patients must have: * Positive toxoplasma serology. * HIV infection. * Willingness and ability to comply with the protocol and capability of giving written informed consent. Exclusion Criteria Co-existing Condition: Patients with the following conditions or symptoms are excluded: * Current diagnosis of cerebral toxoplasmosis or toxoplasmosis infection in any other organ or tissue. * Known or suspected allergy or severe intolerance to study drugs. Concurrent Medication: Excluded: * Anticoagulants. Other antifolates, sulfonamides, fansidar, macrolides, 5-fluorouracil, dapsone, or any other agent with known activity against Toxoplasma gondii.