Despite extensive research, the role of the commonly employed tumour markers in the diagnosis of lung carcinoma is yet to be clarified. The utility of a new marker, CYFRA 21-1, in the preoperative evaluation of patients with bronchogenic carcinoma was investigated. CYFRA 21-1 was determined with a radiometric assay in serum of 280 patients with lung cancer and 208 patients with various nonmalignant lung diseases. The levels of the marker were significantly higher in lung cancer patients. Among benign lung diseases, elevated CYFRA 21-1 levels were found in pulmonary fibrosis. Using a cut-off of 3.2 ng.ml-1 (95th percentile of levels obtained in benign lung disease), the total sensitivity of the marker was 48%. The best sensitivity was obtained in squamous cell lung cancer (60%). The highest values of CYFRA 21-1 were found in metastatic lung cancer, and the marker sensitivity was more elevated in stage IIIb and IV. On the other hand, 40% of patients with surgically resectable lung cancer had CYFRA 21-1 levels above the cut-off. We conclude that CYFRA 21-1 may be satisfactorily employed in the differential diagnosis between malignant and benign lung diseases in association with other clinical and radiological data.