We prospectively evaluated the usefulness of adenosine deaminase [ADA] estimation in the diagnosis of tuberculosis [TB] pleural effusion. Seventy five subjects with pleural effusion were studied. Forty eight of them had TB pleural effusion [M:F: 37:11; mean age 33 +/- 14.4 years range 17-76] and the remaining 27 had pleural effusion due to causes other than TB [non-TB group] [M:F: 19:8; mean age 47.3 +/- 16.5 years; range 17-75]. Pleural fluid [PF] ADA levels were significantly higher in TB (n=48; mean 95.8 +/- 57.5 IU/L) compared with non-TB group (n=27; mean 30.7 +/- 27.2 IU/L) [p<0.001]. Serum ADA [S-ADA] levels were also significantly higher in TB (n=45; mean 39.6 +/- 18.3 IU/L) compared with non-TB group (n=26; mean 18.0 +/- 13.7 IU/L) [p<0.001]. PF-ADA levels were higher compared to S-SDA in TB (p <0.001) and non-TB groups [p<0.01]. Using a cut off of 35 IU/L, the sensitivity and specificity of PF-ADA in the diagnosis of TB was computed to be 83.3% and 66.6% respectively. At a cut-off level of 100 IU/L, PF-ADA was found to have a sensitivity 40% and specificity 100%. From this study it is concluded that, using 100 IU/L as the cut-off, it is possible to avoid pleural biopsy to ascertain the diagnosis of TB in as much as 40% of the patients.