Objective: Tuberculous meningitis (TB M) is the commonest form of neurotuberculosis caused by Mycobacterium tuberculosis bacilli (MTB). The diagnosis of TBM is often difficult. A reliable, cost effective and rapid diagnostic test, which can be performed in any routine laboratory, could be of help in the diagnosis of TBM. In the present study we measured the adenosine deaminase (ADA) activity in cerebrospinal fluid (CSF) of TBM and non-TBM patients. Method: ADA activity in CSF was determined according to a method based on the modified Berthlot reaction, which is the formation of a colour indophenol complex from ammonia liberated from adenosine, and quantified spectrophotometrically. Results: ADA activity was studied in CSF of 45 cases of tuberculous meningitis, 20 cases of pyogenic meningitis and 20 controls (patients without any neurological disorders who were given spinal anaesthesia). The mean ADA activity was 13.43 ± 4.76, 8.23 ± 2.91 and 2.64 ± 1.57 IU/L in tuberculous meningitis, pyogenic meningitis and controls respectively. The higher level of ADA in tuberculous meningitis was statistically significant (p < 0.0001). The sensitivity and specificity of ADA test in CSF at cut-off 10 U/L was 100% and 93.75% respectively. Conclusion: This study demonstrated that adenosine deaminase activity in CSF, a relatively inexpensive and easy procedure, can be of great value in the diagnosis of tuberculous meningitis. This test can be performed in any routine laboratory where more sophisticated methods are not available.