Cerebrospinal fluid (CSF) was analysed in 50 AIDS patients in this investigation. CSF analysis included: pressure, aspect, colour, cytology (cell number, cytomorphology, and lymphocyte subpopulations), proteins concentration and their electrophoretic distribution, chloride and glcosis concentration, GOT and LDH activity, immunology for cysticercosis, syphilis and toxoplasmosis, bacteriology, and mycological examinations (direct, cultures, Cryptococcus neoformans antigen test). Diagnostic changes were: lymphoma cells in one patient, cryptococcosis in 10, toxoplasmosis in 4, candidiasis in 1, and syphilis in 3. Cryptococcosis occurred in association with toxoplasmosis in one patient who developed syphilis in the follow-up; oligoclonal distribution of gamma globulins ocurred in this case. In another case of cryptococcosis, candidiasis appeared during the course of the disease. Changes in the CSF composition observed are discussed in order to evaluate: aspects of CSF inflammatory response in report to immunodeficiency; blood-brain barrier involvement; CNS damage; intracranial mass effects.