CYFRA 21-1 is a new tumor marker using two different monoclonal antibodies which recognize the divergent epitope on the N- or C-terminal region of domain 2 of cytokeratin 19 fragment, respectively. In this study, we investigated the relationship between levels of CYFRA 21-1 and survival duration, as well as the efficacy of chemotherapy associated with changes in CYFRA 21-1. Serum samples were obtained from 87 patients with nonoperable lung cancer (35 cases with squamous-cell carcinoma, 33 with adenocarcinoma, 3 with large-cell carcinoma, and 16 with small-cell carcinoma). The cutoff point was set at 3.5 ng/ml. In a CYFRA 21-1 assay, significantly more patients with squamous-cell carcinoma and adenocarcinoma were positive compared to patients with small-cell and large-cell carcinomas (p = 0.0017). Following chemotherapy, blood levels of CYFRA 21-1 decreased significantly in responders versus nonresponders (p = 0.0246). A significant correlation was noted between survival periods and pretreatment levels of CYFRA 21-1 (p = 0.0036). The present study suggests that CYFRA 21-1 might be useful as a possible indicator of survival and therapeutic effect for lung cancer.