Cytokeratin-19, one of the cytoskeletal proteins, is expressed both in bronchial epithelium and in lung cancer cells. The aim of our study was to establish the value of serum cytokeratin-19 soluble fragment (Cyfra 21-1) measurement in lung cancer patients. Cyfra 21-1 levels were estimated in 35 patients (pts) with benign lung diseases and in 116 lung cancer patients: 55 pts with squamous cell lung cancer, 38 pts with small cell lung cancer and 23 pts with adenocarcinoma. The cutoff level was set at 4 ng/ml with a specificity of 94% and a sensitivity of 40%. Elevated Cyfra 21-1 values were found in 44% of squamous cell lung cancer, 39% of adenocarcinoma and 34% of small cell lung cancer pts (the difference was not significant). In squamous cell lung cancer and in adenocarcinoma elevated Cyfra 21-1 values were observed more often in patients with advanced disease than in patients with limited disease. There was no significant correlation between the initial Cyfra 21-1 level and the response to chemotherapy. Cyfra 21-1 was not a prognostic indicator, although in operable squamous cell lung cancer the proportion of survivors in the second year of observation was higher among the patients with normal preoperative Cyfra 21-1 levels.