The aim of this study was to determine whether estimation of sialic acid /el'els, both in the serum and pleura/fluid, of patients with pleural effusion is of significance in detecting malignant pleural effusions. This study included 50 patients with p(eural effusion, 20 patients with malignant effusion (group/) and 30 patients with non malignant effusion (group II). Group II was subdivided into group 11-a included 10 patients with tuberculous effusions, group •li-b included 10 patients with parapneumonic effusion and group 11-c included /0 patients with transudative effusion. Group l/1 of 20 apparently healthy subjects as a control group was also included. The mean sialic acid level in the serum of the control group was 6.13 ± 1.87 mgldl, in the non malignant group it was /0.04 ± 3.18 mgldl and in the malignant group it was 18.85 ± 7.34 mgldl (P < 0.0005). The mean sialic acid lel'el in the pleural fluid of group II (non malignant group) was 5.6 ± 2.42 mgld/while in group I (maligncmt group) was 9.16 ± 3.08 mgldl (P < 0.0005). It can be concluded that, there is significant associatio11 between malignancy and raised sia lic acid in the serum and pleural fluid, thus estimation of sialic acid level may be helpful as a tumour marker in the diagnosis of malignant pleural effusion.