Although the pleural cavity contains a small amount of pleural fluid in normal conditions the amount of fluid increases up to a few liters (an effusion) during pathological changes. The differentiation of the pleural effusion as being a transudate or exudates is the first step in the diagnosis. The aim of this study is to distinguish transudative pleural fluid from exudative one by measuring the LDH, cholesterol and bilirubin levels in effusion and serum to improve the accuracy of pleural fluid categorization. This study was conducted on 30 patients with clinical and radiological evidence of pleural effusion from chest, tropical and cardiology departments Kasr EL-Aini Hospital and Chest Giza Hospital. It was found that there is significant difference between exudates and transduates according to bilirubin, cholesterol and LDH levels in effusion but not the same significant difference between different causes of transduates and exudates.