During a 40-month period 78 episodes of encephalic toxoplasmosis (ET) were diagnosed in 57 patients among 394 with acquired immunodeficiency syndrome (AIDS) (based on CDC criteria) in the Hospital Clinic from Barcelona. 38 patients were parenteral drug abusers (66.7%), 17 were homosexual males (29.8%) and 2 were heterosexual females (3.5%). ET was the first opportunistic infection in 28 patients (49.1%). The most common symptoms at the time of diagnosis were focal neurological signs (62.8%) and fever (52.6%). Usually, computed tomography showed single or multiple hypodense lesions (70.3%) with ring-like (46.1%) or nodular (28.3%) uptake. Serology (indirect immunofluorescence) was diagnostic in only 24 cases (42.1%). The acute episodes were treated with the combination pyrimethamine/sulfadiazine (P/S) during 3-6 weeks, and folinic acid was associated. If the patient had previous allergy to sulfonamides or allergy developed during the acute phase, clindamycin (C) was given instead of S. The patients surviving the acute episode (49, 86, 0%) underwent, if they consented, maintenance therapy with P/S two days per week (15 patients) or with C if sulfonamide allergy was present (10 patients). No relapse was observed in those receiving P/S, but 40% of those treated with P/C relapsed. It was concluded that ET is a common opportunistic infection in the AIDS patients, that the treatment of the acute phase with P/S or with P/C is satisfactory, and that maintenance therapy with P/S two days per week may be effective to prevent relapses.