BACKGROUND: Cell mediated immune response mediated by interaction between CD4+ T lymphocytes and macrophagies is thought to play an important role in tuberculous pleurisy. This interaction is dependent on the interplay of various cytokines. The immunologic response of tuberculous pleurisy is thought to depend on the balance between helper T cell(Th1) cytokine Interleukin-12, Interferon gamma and Th2 cytokine IL-4, IL-10. To understand immunologic mechanism in tuberculous pleurisy and evaluate diagnostic value of these cytokines, the concentrations of Th1 cytokine IL-12, IFN-gamma and Th2 cytokine IL-10 were measured in tuberculous pleurisy and malignant pleural effusion group. MATERIAL AND METHODS: The concentrations of IL-b, IL-12 and IFN-gamma were measured by ELISA method in pleural fluids and serums of 20 patients with tuberculous pleurisy and 20 patients with malignant pleural effusion. ADA activities were measured by spetrophotomery in pleural fluids of both groups. RESULTS: In tuberculous pleurisy, the mean concentrations of IL-b, IL-12 and IFN-gamma of pleural fluids showed 121.3+/-83.7 pg/mL, 571.4+/-472.7 pg/mL and 420.4+/-285.9 pg/mL. These were significantly higher than that of serum, 21.2+/-60.9 pg/mL, 194.5 pg/mL, 30.1+/-18.3 pg/mL respectively(p<0.0l). In malignant pleural effusion, the mean concentrations of IL-10, IL-12 and IFN-gamma of pleural fluids showed 88.4+/-40.4 pg/mL, 306.5+/-271.1 pg/mL and 30.5+/-54.8 pg/mL respectively. Compared with that of serum (43.4+/-67.2 pg/mL, 206.8+/-160.6 pg/mL, 14.6+/-3.3 pg/mL), only IL-10 was significantly higher (p <0.001), but IL-12, IFN-gamma were not significant. In tuberculous pleural effusion compared with malignant pleural effusion, the concentration of IL-12, IFN-gamma, ADA were significantly higher (p value 0.046, <0.001, <0.001), but IL-10 was not significant. For differetial diagnosis of tuberculous pleurisy from malignant pleural effusion, using cut-off value of IL-12, IFN-gamma ADA as 300 pg/mL, 100 pg/mL, 45 U/L, the sensitivity/specificity were 60%/70%, 90%/87.5%, 85%/90% respetively. CONCLUSION: In tuberculous pleurisy, IL-10, JL-12 and IFN-gamma were selectively concentrated highley in pleural space than serum. Compared with malignant pleural effusion, IL-12 and IFN-gamma were significantly higher, but IL-10 were not in tuberculous pleural effusion. The results suggest that Th1 pathway contributes to immune resistant mechanism in tuberculous pleurisy. IFN-gamma and ADA revealed useful methods of differential diagnosis in tuberculous pleurisy from malignant pleural effusion.