STUDY OBJECTIVE:To study the value of adenosine deaminase level in tuber-cular pleural effusion.DESIGN:A hospital based observational study.SETTING:Out and In patients service of department of Tuberculosis & Chest Diseases, MLN Medical College Allahabad.PATIENTS:50 consecutive patients of pleural effusion, who were above the age of 12 years, were studied.RESULTS:Pleural fluid adenosine deaminase was more than 36 IU/L (36 to 229.7 IU/L) in tubercular pleural effusion (34 patients). In case of malignancy no. of patients was 08 and pleural fluid adenosine deaminase was more than 18.5 IU/L (18.5 to 87.6 IU/L). While in one case of hypoprotenemiea pleural fluid adenosine deaminase was 8.21 IU/L. If 36 IU/L is taken as cut of limit the sensitiv-ity and specificity of ADA for tuberculosis is 100 % and 77.7 %. More than 100 IU/L was exclusively seen in tubercular pleural effusion.CONCLUSION:ADA > 100IU/L was observed in TB only.