Background: Meningitis is inflammation of the meninges (pia, arachnoid and duramater) covering the brain and the spinal cord. The most common cause of meningitis is infections due to viruses, bacteria, mycobacteria, fungi and other microorganisms. Objective: To estimate the level of CSF Adenosine Deaminase level in different types of meningitis & To assess its usefulness in differentiating the various types (bacterial, viral and tuberculous) of meningitis. Design & Methodology: A prospective study conducted in 50 consecutive patients with signs and symptoms of meningitis in Meenakshi Medical College Hospital & research Institute, Kanchipuram. The different types of meningitis patients were separated on the basis of CSF cytochemistry and clinical features. Adenosine Deaminase(ADA) activity was determined in cerebrospinal fluid(CSF) by the method of Guisti and Galanti based on the Berthlot reaction that is the formation of coloured indophenol complex from ammonia liberated from adenosine and quantified spectrophotometrically. Results & Conclusion: The primary outcome studied was the CSF ADA level in various types of meningitis and comparison of the level of elevation in Tuberculous with other types of meningitis. Cases of tuberculous meningitis constituted 48% of the study group and bacterial and viral meningitis were 26% each. CSF ADA is a simple, time saving, inexpensive indirect test that helps in identifying the type of meningitis, differentiating tuberculous from nontuberculous etiology. Further among the nontuberculous group of meningitis, ADA values are lowest in viral meningitis and thus it can aid in distinguishing bacterial from viral etiology. CSF ADA cut–off of 10U/L considered in our study has 100% sensitivity and 92.85% specificity in differentiating tuberculous from nontuberculous etiology as early as possible, facilitating immediate management of these patients preventing morbidity and mortality due to delay in management.