Abstract： @@ The proportion of tuberculous pleurisy among all pleurisy cases is 49.6% and is obviously increasing. However, malignant pleural effusion (29.6% of cases) also accounts for a significant percentage of pleural diseases. Therefore, tuberculous pleurisy and malignant pleurisy are two main causes for pleural effusion.1 Because of their quite different prognosis and therapy, it is very important to differentiate them clearly. However, occasionally it is difficult to confirm the diagnosis. Determination of adenosine deaminase (ADA) in pleural effusion has been confirmed as a useful supplemental diagnostic index for tuberculous pleurisy. However, ADA2 as a major isoenzyme increasing in tuberculous pleurisy has not been well investigated in China so far. On the other hand, interferon-γ (IFN-γ) in tuberculous pleural effusion had been considered as a diagnostic marker of tuberculous pleurisy.2,3 In this study, the concentration of IFN-γ and interlukin-12 (IL-12), as well as the activity of ADA and ADA2 in pleural effusion were measured in order to investigate their changes in tuberculous and malignant effusion, and find out the best marker for diagnosis of tuberculous pleurisy.