The Cyfra 21.1 assay is a newly developed test which measures in serum a fragment of cytokeratin 19. We evaluated this marker in 212 patients with non-small-cell lung cancer (NSCLC), predominantly stage 3a-b and 4, and compared it with three other markers: carcinoembryonic antigen (CEA), squamous cell carcinoma antigen (SCC) and tissue polypeptide antigen (TPA). Sensitivities for Cyfra 21.1, TPA, CEA and SCC (using cut-off levels corresponding to a 95% specificity for benign lung diseases) were 40%, 40%, 42% and 19% respectively. The sensitivity of CEA was significantly higher in patients with adenocarcinomas compared with the other three markers, while the sensitivity of Cyfra 21.1 and TPA was significantly higher in patients with squamous cell carcinomas. The value of Cyfra 21.1 for monitoring disease during chemotherapy could be evaluated in 23 patients with squamous cell carcinomas. When the cases of lead time were included a concordance between clinical evaluations according to WHO response criteria and evaluations according to changes in the marker levels of 74% was found. The criteria defined for marker response were a 65% decrease in the marker level for a partial response and a 40% increase for progressive disease. In particular, increasing levels of this marker indicated usually disease progression. In conclusion, Cyfra 21.1 is a useful serum marker for patients with NSCLC, especially for disease monitoring of patients with squamous cell carcinoma during and after chemotherapy.