In this study, the clinical significance of serum carcinoembryonic antigen (CEA) level in small cell lung cancer patients was investigated. The relationship between serum CEA level before treatment and the effect of chemotherapy was analyzed. In 97 evaluable patients with small cell lung cancer, the 26 who had elevated CEA values of 10.0 ng/ml or greater at diagnosis tended to be resistant to intravenous systemic chemotherapy. In patients with limited disease (LD), survival time of 13 patients whose CEA levels were 10.0 ng/ml or greater was shorter than that of the other LD patients. These findings suggest that small cell lung cancer patients with high serum CEA levels are less responsive to standard chemotherapy, and serum CEA level may be able to identify biologically different tumors from common small cell cancer.