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Adenosine deaminase enzyme (ADA) activity was estimated in 73 patients of pleural effusion. Patient were classified into five different groups based on aetiological diagnosis made on pleural biopsy and fluid examinations. ADA concentrations were significantly higher in tubercular effusion. Four of 26 patients (15.4%) with malignant pleural effusion had also shown raised ADA levels. All these patients had predominantly lymphocytic effusion. Though a higher ADA value is strongly suggestive of tubercular effusion, malignancy cannot be altogether excluded. However, ADA activity less than 40 IU/L excludes tubercular aetiology.