Different biochemical tests including total protein, LDH, Bilimbin, cholesterol, ADA and glucose were measured in 40 patient with pleural effusion. Five (12.5%) out of 40 had heart failure, 2 (5%) have chronic renal failure, 2 (5%) suffered from diabetic nephropathy, 15 (37.5%) with tuberculosis, 4 (10%) with empyema, 2 (5%) with pneumonia, 9 (22.5%) had malignancy and one (2.5%) with traumatic haemothorax. Patients were classified as follows: 1- Patients with transudative or exudative eflusion depending on total protein, LDH. bilirubin and cholesterol measurements. 2- Patient with tuberculosis, empyema (group 1) and those with carcinooma, CHF, and pneumonia (group 2) depending on ADA test (ADA ratio 2.5). 3- Patient with tuberculosis pleural effusion and those with malignant effusion depending on AD A test (AD A ratio 1.4). Significant increase in pleural fluid to serum total protein (P<0.009), LDH r 0.0001), bilirubin (PO.0001) ratios and pleural fluid cholesterol concentration (PO.002) were found in patient with exudative effusion as compared with transudative effusion. Highly significant difference was observed in ADA ratio between patients withtuberculosis and ernpyema and those with carcinoma. CHF and pneumonia (P<0.0001), on the other hand, significant difference was found in .ADA ratio between patients with tuberculosis pleural effiision and malignant effusion. In contrast to prevalent opinion that glucose level must be decreased in exudative effusion, pleural fluid glucose level was high rather than low in patients with tuberculosis (mean 86mg/dl), and carcinoma (mean 85mg/dl), on The other hand, glucose level in patients with pneumonia was low (mean 53mg/dl), and in empyema it was very low (mean 24mg/dl). Significant correlation were observed between PF/S total protein and LDH (r= 0.374, P<0.01 ), further more significant correlation were found between PF/S bilirubin ratio and PF/S total protein (r= 0.551, P<0.0001), LDH (r=0.473 P<0.001) ratios respectively. No significant correlation was found between PF/S total protein ratio and PF total cholesterol level, also no significant correlation was noted between PF/S LDH ratio and PF total cholesterol level.