Abstract: Tuberculous pleurisy and malignancy are two of the most common causes of pleural effusion, but are difficult to differentiate between them in certain clinical situations. IL-33 is mainly expressed in lining epithelial and endothelial cells and is released after cell damage; it is proposed to have an important role in sensing damage in various infectious and inflammatory diseases. This work aimed to study the diagnostic role of IL-33 in the differentiation between tuberculous and malignant pleural effusion. Quantitative measurement of IL-33 using ELISA technique in pleural effusion and serum samples in 12 cases with TPE, 12 cases with MPE, 8 cases with parapneumonic pleural effusion and 8 cases with transudative pleural effusion. The concentration of IL-33 (mean ± SD) in the TPE patients (22.5±0.90ng/l) was significantly higher than that in the MPE patients (14.6±2.35ng/l; P0.05). The concentration of IL-33 in the pleural effusions was positively correlated with that in the serum samples in each group (TPE: r=0.848, P=