The aim of this retrospective study was to define a diagnostic strategy and to evaluate the efficacy of cotrimoxazole (CTMX) for presumed cerebral toxoplasmosis in patients with AIDS. Twelve patients with toxoplasma encephalitis were reviewed. The best diagnostical signs of reactivated acute cerebral toxoplasmosis were the association of neurological symptoms indicative of focal cerebral lesions, and a radiological picture showing ring contrast enhanced hypodense mass-lesions; serology was unreliable for the diagnosis. Five patients out of twelve were treated without delay and until death with CTMX. Only these improved their clinical and radiological status obviously. Moreover, their median survival time was clearly longer (160 days, versus 9 days) and their autopsy demonstrated the absence of active necrotizing lesions of toxoplasma encephalitis. So, CTMX seems to be an efficient therapy for suspected cerebral toxoplasmosis in AIDS. Nevertheless, further prospective randomized therapeutic trials are required to confirm this impression.