The serum adenosine deaminase (ADA) level was estimated in 30 patients with pleural effusion and 18 healthy controls and was found to be significantly elevated in the former (P less than 0.001). The pleural fluid ADA level in tubercular pleural effusion was always above 50 U/L and was significantly higher than in nontubercular effusions (P less than 0.001). Pleural fluid ADA to serum ADA ratio was not helpful in the aetiologic diagnosis of pleural effusions. Thus, estimation of pleural fluid ADA is a safe, simple, reliable and non-invasive test in distinguishing tubercular and non-tubercular pleural effusions.