The aim of this study was to evaluate the individual and combined diagnostic utility of six tumor markers in patients with pleural effusion. Pleural and serum levels of carcinoembryonic antigen (CEA), carbohydrate antigen 15-3 (CA 15-3), carbohydrate antigen 19-9 (CA 19-9), cytokeratin fragment 19 (CYFRA 21-1), neuron-specific enolase (NSE) and total sialic acid (TSA) were assayed in 74 patients with pleural effusions (44 malignant and 30 benign). All tumor markers except TSA and NSE were increased in both serum and pleural fluid of patients with malignant diseases. Using the cut-off values 3 ng/ml, 14 U/ml, 5 U/ml, 8 ng/ml and 70 mg/dl for pleural fluid CEA, CA 15-3, CA 19-9, CYFRA 21-1 and TSA, respectively, the sensitivity (%) and specificity (%) of these tumor markers were as follows: CEA; 52/77, CA 15-3; 80/93, CA 19-9; 36/83, CYFRA 21-1; 91/90, TSA; 80/67, for differentiating malignant effusions from benign. When CA 15-3 and CYFRA 21-1 combined, the sensitivity and specificity were increased (100 and 83%, respectively). Classifying the malignant effusions as bronchial carcinoma and malignant pleural mesothelioma, CEA was shown to have the highest sensitivity and specificity (88 and 90%, respectively) while the combination of CEA with other tumor markers increased sensitivity but decreased specificity. According to our results, tumor markers are not suitable for the differential diagnosis of malignancy.