Cytokeratin 19 is a subunit of cytokeratin intermediate filament expressed in simple epithelia and their malignant counterparts. Therefore, it is expressed by respiratory epithelium cells and has been detected in lung cancer specimens. An immunoradiometric assay was used to detect a fragment of the cytokeratin 19, referred to as CYFRA 21-1, in the serum of 165 patients with histologically proved lung cancer (128 non-small cell and 37 small cell lung cancers). This prospective study was conducted to evaluate the reliability of this immunoradiometric assay and to identify the relationship between serum CYFRA 21-1 and different features of lung cancer including prognosis. The minimal detectable concentration detected by this assay was 0.06 ng/ml. The reliability of the immunoradiometric assay was demonstrated by the linear relationship between CYFRA 21-1 measurement and dilution of the serum, the reproducibility of the dosage in intraassay and interassay, and the high sensitivity of the method in discriminating low CYFRA 21-1 concentrations. Using a threshold of 3.6 ng/ml, sensitivity and specificity were 0.52 and 0.87, respectively. The sensitivity of the marker was highest in squamous cell carcinoma and lowest in small cell carcinoma. In non-small cell lung cancer patients, the marker varied significantly according to both stage of the disease (Kruskal-Wallis, 13.7; P < 0.005) and performance status (Kruskal-Wallis, 9.16; P < 0.05) inasmuch as a high serum CYFRA 21-1 level was associated with advanced stages, mediastinal lymph nodes, and poor performance status. Consequently, the marker was significantly lower in patients who were operated upon when compared with unresectable ones. Lung cancer patients with serum CYFRA 21-1 over 3.6 ng/ml proved to have a significantly shorter overall survival than those with a normal serum level (log rank, P = 0.007; Wilcoxon, P = 0.001). The negative prognostic effect of CYFRA 21-1 was highly significant in squamous cell carcinomas whereas it was nonsignificant for the other histologies. In Cox's model analysis, performance status, stage grouping, and CYFRA 21-1 were the only significant determinants of survival. This study supports the use of the serum fragment of cytokeratin subunit 19 CYFRA 21-1 as an independent prognostic marker of squamous cell carcinoma of the lung.