Abstract: Objective To assess the diagnostic value of adenosine deaminase(ADA) activity in the cerebrospinal fluid of tuberculous meningitis patients. Methods The ADA enzyme was measured spectrophotometrically to determine the ADA activity in the cerebrospinal fluid of 102 patients, who were divided into four groups (30 with tuberculous meningitis, 34 with bacterial meningitis, 15 with viral meningitis, and 23 with mere headaches). Results The ADA activity was significantly elevated in the cerebrospinal fluid of 30 patients with tuberculous meningitis, of which 26 had the ADA greater than 6.0 U/L, indicative of statistical difference as compared to that of patients with bacterial meningitis (P0.01), the sensitivity being 86.7% (26/30) and the specificity 91.7% (66/72). Statistical differences were also noticed between the ADA activity in the cerebrospinal fluid of patients with bacterial meningitis and that of those with viral meningitis and of those with mere headaches (as the control group, P0.05). Conclusion The ADA activity in cerebrospinal fluid is of great value in the diagnosis and differential diagnosis of tuberculous meningitis.