BACKGROUND: It is sometimes difficult to differentiate tuberculous pleural effusion from malignant pleural effusion by clinical symptoms, signs, by routine tests of pleural fluid, and by pathologic studies. And recently, it was discovered that cytokines such as IL-2, IFN-gamma TNF-alpha are elevated in tuberculous pleural fluid, and there have been several attempts to diagnose tuberculous pleural effusion by using these immunological mediators. There are several studies regarding the diagnostic value of IFN-gamma, and there are two studies in Korea. But the diagnostic values of IFN-gamma in these studies were slightly lower than those in other countries. To compare the diagnostic value of IFN-gamma with those of CEA and ADA, and to determine the sensitivity and specificity of IFN-gamma in Korean, we mesured IFN-gamma, CEA level and ADA activity in pleural effusions. METHODS: ADA activity, IFN-gamma level and CEA level as well as cell count, differential count, and biochemical assays such as protein content and lactate dehydrogenase were measured in 40 cases of tuberculous pleuritis and 42 cases of malignant pleural effusion. RESULTS: Tuberculous pleural fluid showed higher levels of IFN-gamma and ADA (832.6+/-357.2 pg/ml and 82.5+/-25.9 U/L, respectively) than those of malignant pleural effusion (2.6+/-8.0 pg/ml and 19.2+/-10.9 U/L, respectively) (p<0.01). Malignant pleural effusions showed higher median value (102.2 ng/ml) than tubercalous pleural effusions (1.8 ng/ml) (p<0.01). The sensitivities of IFN-gamma, ADA, CEA were 0.97, 0.87, 0.67 and the specificities of IFN-gamma, ADA, CEA were 1.0, 0.97, 1.0, respectively. There was no significant correlation between ADA activity and IFN-gamma level. CONCLUSION: This study showed that IFN-gamma test would be a very useful clinical test for differential diagnosis of tuberculous pleuritis and malignant pleural effusion because it is very sensitive and specific, although it is an expensive test.