BACKGROUND: The aim of the present study was to evaluate the diagnostic value of the tumour markers carcinoembryonic antigen (CEA), carbohydrate antigens CA 125, CA 15.3, CA 19.9 and tumor-associated glycoprotein 72 (TAG 72) in the pleural fluid (PF) of patients with pleural effusions of different etiologies. PATIENTS AND METHODS: One hundred and fifty-five patients with pleural effusions (40 malignant, 84 benign and 31 paraneoplastic) were studied prospectively. The concentration of the tumour markers in serum and PF were measured by magnetic particle enzyme immunoassay. The PF to serum (PF/S) concentration ratios were calculated. RESULTS: The concentrations of CEA, CA 15.3, CA 19.9 and TAG 72 in PF and the PF/serum ratios were significantly higher in effusions of malignant and paraneoplastic origin than in those of benign origin. The receiver operating characteristic (ROC) curves were calculated for each marker and the diagnostic cut-off point was selected as the value that offered a specificity of 100% (CEA: 6.5 ng/ml; CA 15.3:62.4 IU/l; TAG 72:10.9 IU/l). CEA presented the greatest sensitivity [45% in the malignant group, 38.7% in the paraneoplastic group, and 41.4% in the pooled group (combined malignant and paraneoplastic)]. TAG 72 presented the largest area under the curve (0.89 in the malignant group and 0.80 in the pooled group). The diagnostic efficacy of the PF/S ratios was not better than measurement of the tumour markers in pleural fluid. The highest diagnostic accuracy for the diagnosis of malignant pleural effusions was achieved by grouping the markers in a panel comprising CEA, CA 15.3 and TAG 72; this showed a sensitivity of 75% and a negative predictive value of 79.1% . In the subgroup of patients with negative cytology, the sensitivity was 41.2% for CEA, 35.5% for CA 15.3 and 33.3% for TAG 72. The combination of these three markers achieved a sensitivity of 84.6%. CONCLUSION: The combined measurement of CEA, CA 15.3 and TAG 72 in pleural fluid is a useful complementary test in the differential diagnosis of pleural effusions of malignant origin.