STUDY OBJECTIVE: To study the diagnostic value of ADA level in various etiology of pleural effusion. To find out sensitivity, specificity, positive predictive value & negative predictive value of ADA in diagnosis of pleural effusion. DESIGN: A hospital based observational study. PATIENTS: 50 consecutive patients of pleural effusion, who were above the age of 18 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 68 IU/L). While in one case of hypoproteinemia pleural fluid adenosine deaminase was 13.2 IU/L. If 36 IU/L is taken as cut of limit the sensitivity and specificity of ADA for tuberculosis is 97.06 % and 75 %. More than 100 IU/L was exclusively seen in tubercular pleural effusion. CONCLUSION: ADA > 100IU/L was observed in TB only.