Relapse of brain toxoplasmosis in AIDS patients continue to occur despite maintenance therapy. The characteristics of 15 patients who experienced a relapse were reviewed. Mean delay of relapse was 7 months (2-20). Maintenance therapy for the 2 months prior to the relapse was pyrimethamine alone (n = 4), pyrimethamine plus dapsone (n = 3), pyrimethamine plus spiramycine (n = 1), pyrimethamine plus clindamycine (n = 4), pyrimethamine plus sulfadiazine (n = 1) and no treatment (n = 2). Strict compliance was ascertained for only 5 patients. Compared with the first episode of brain toxoplasmosis, all 15 patients had new neurological signs, and new lesions of the brain were detected by CT scan in 67% patients. This study showed that poor compliance to maintenance therapy is a major factor of relapse, and the less favourable response to acute therapy could be due to progression of immunodeficiency.