Cytokeratin 19 is particularly abundant in carcinoma of the lung. The CYFRA 21-1 assay has recently been developed for detection of a cytokeratin 19 fragment in serum. In the current study, the prognostic information provided by the CYFRA 21-1 assay in operable nonsmall cell lung cancer (NSCLC) was analysed. Serum levels of CYFRA 21-1 were measured using an immunoradiometric assay (DiaSorin) in 94 patients with operable NSCLC. Survival and disease-free survival curves related to initial levels of this marker were estimated using the Kaplan-Meier method. Elevated preoperative CYFRA 21-1 levels were identified in 42% of patients with NSCLC. The number of patients with elevated levels of this marker increased with tumour node metastasis (TNM) stage (p=0.02). In univariate analysis elevated levels of CYFRA 21-1 were significantly associated with poor overall survival (p<0.001) and with disease-free survival (p<0.001). The results remained significant when the comparisons were adjusted, using the stratified log-rank test, for patient's TNM stage (p<0.001 for both overall and disease-free survival). Elevated preoperative levels of CYFRA 21-1 decreased the probability of survival or surviving without recurrence 15 months or more after the operation. This was confirmed by the results of the multivariate analysis. In conclusion, CYFRA 21-1 may be an independent prognostic parameter of survival and tumour relapse in nonsmall cell lung cancer and may be useful in identifying resected cancer patients at high risk for treatment failure.