STUDY OBJECTIVES: Pleural tuberculosis (TB) is a diagnostic challenge because of its nonspecific clinical presentation and paucibacillary nature. The inefficiency of conventional laboratory methods and the reliance on pleural biopsy have motivated the evaluation of alternative diagnostic strategies. We have evaluated polymerase chain reaction (PCR) directed to the IS6110 sequence of Mycobacterium tuberculosis, the determination of adenosine deaminase (ADA) activity, and measurement of interferon (IFN)-gamma levels in pleural fluid in the diagnosis of pleural TB. PATIENTS: ADA activity, IFN-gamma levels, and PCR were evaluated in 140 cases of pleural effusion, 42 with confirmed pleural TB, 19 with probable pleural TB, 70 with a nontuberculous etiology, and 9 having an undetermined etiology. RESULTS: ADA activity, IFN-gamma levels, and PCR were 88%, 85.7%, and 73.8% sensitive, respectively, and 85.7%, 97.1%, and 90% specific, respectively, for pleural TB that had been confirmed by either culture or pleural biopsy specimens. The combination of PCR, IFN-gamma measurement, and ADA activity determination allowed the selective increase of sensitivity and specificity for probable and confirmed cases compared to individual methods. Positive and negative predictive values for these individual or combined methods were maintained over a wide range of prevalence of pleural TB in the patient population presenting with pleural effusions. Fever and younger age were associated with tuberculous pleural effusion (p < 0. 0001), while blood in sputum and older age were associated with malignant etiology (p < 0.008). CONCLUSIONS: These clinical variables together with the use of ADA activity determination, PCR, and measurement of IFN-gamma levels provide the basis for the rapid and efficient diagnosis of pleural TB in different clinical settings.