We studied the activity of adenosine deaminase in the cerebrospinal fluid of 40 normal controls and 205 patients who were grouped according to disease (tuberculous, viral, and purulent meningitis; neoplasms; stroke; and miscellaneous). The mean enzyme value was clearly higher for the patients with tuberculous meningitis (15.7 ± 4.3 U/liter) than for the other patients (1.4 ± 1.5 U/liter). The sensitivity of the test for diagnosing tuberculous meningitis was 1 and specificity, 0.99. The enzyme activity, as well as progression of the disease, was studied in 32 patients with tuberculous meningitis. A significant rise in levels of enzyme was observed during the first 10 days of therapy, was followed by a gradual decline, and reached normal values after three to four months of treatment. Two patients showed substantial increases that coincided with the development of complications. The test proved to be a simple and reliable method for early diagnosis and follow-up of tuberculous meningitis. © 1987 by The University of Chicago.