The present study was conducted in 30 patients of malignant pleural effusion and 30 patients of non malignant pleural effusion. Pleural fluid and blood samples were taken at the time of admission, before starting any treatment. Sialic acid levels were estimated in serum and pleural fluid by Warren's TBA method. In the present study, serum sialic acid levels were higher in group II as compared to group I. In the present study, pleural fluid sialic acid levels and PF/S ratio was higher in malignant pleural effusion (though difference was not statistically significant). Smokers in group II had higher serum sialic acid as compared to group 1 (P < 0.05). The sensitivity and specificity of pleural fluid/serum sialic acid ratio with cut off value of 0.7 were 76.67% and 20% respectively, while taking the cut off value of 70 mg/dL for pleural fluid sialic acid in malignant pleural effusions, the sensitivity was 63.33%, specificity 60% and positive predictive value 46.34%. These findings indicate that determination of sialic acid levels in pleural fluid has diagnostic value as a cheap, simple and reliable marker for malignant pleural effusion.