The adenosine-deaminase (ADA) activity was evaluated in CSF samples from 263 patients with AIDS. An elevated ADA activity in CSF was found in patients with: antibodies to toxoplasmosis, syphilis or cytomegalovirus; Cryptococcus neoformans or their antigens; tuberculous meningitis; lymphoma. There was no statistical difference among all these groups in respect to ADA activity. However, the ADA activity in CSF from AIDS patients without CSF changes other than HIV antibodies, even unspecific changes, was not elevated. This may suggest that ADA is related to AIDS associated pathologies activity rather than to HIV infection itself.