Nineteen male patients with AIDS were investigated by biopsy of brain lesions. Six patients had progressive multiple leucoencephalopathy and no specific treatment was given. Toxoplasmosis (two patients), bacterial abscesses (two cases), viral encephalitis (two patients) and only gliosis (two cases) accounted for almost half of the cases. A fungal infection, a lymphoma and a sarcoma-like tumour were found in three patients. In two patients the biopsy was not diagnostic: one had only necrosis and one had normal findings. The biopsy findings gave reason for modifying the treatment in only three cases. The mean survival rate was relatively short, only 76 days with a range from 1 to 1041 days. Two patients were in a very bad clinical condition at the time of biopsy and one died of a haemorrhagic complication due to the biopsy. In ten cases an autopsy was carried out. In five cases there was agreement between the biopsy and autopsy findings. In the other five cases the autopsy findings differed widely. In our experience we can recommend cerebral biopsy in patients with AIDS only after treatment for toxoplasmosa and mainly to estimate the prognosis.