Central nervous system involvement occurred in 28 of 121 patients with acquired immune deficiency syndrome (AIDS). The major risk factor in this AIDS population was intravenous drug abuse (64%). A neurologic symptom or disability was the principal reason for hospitalization in 16 cases (57%). Three patients had primary lymphoma of the brain and the remainder had opportunistic infections. Patients with focal neurological features usually had toxoplasmosis. Progressive headache and meningeal signs occurred with cryptococcosis. A progressive subacute dementia was probably due to cytomegalovirus. Other infections included atypical mycobacteria, candida, herpes zoster and possible progressive multifocal leukoencephalopathy.