The clinical value of the serum biomarker carcinoembryonic antigen (CEA) was evaluated prospectively in 118 patients with small cell lung cancer (SCLC) entered chemotherapy protocol between 1986 and 1992. Five quantitative categories were determined: less than 2.5 ng/ml and 2.6-5.0 ng/ml (the standard normal), 5.1-20.0 ng/ml, 20.1-100 ng/ml and greater than 100 ng/ml. 70% of patients had levels less than 5 ng/ml and only 19% had levels greater than 20 ng/ml. There was no clearcut relationship of plasma CEA level to stage of disease, in which 61% of patients with extensive disease (59 patients) had levels less than 5 ng/ml and 22% of patients with limited disease (59 patients) had levels greater than 5 ng/ml. There was a modest relationship of CEA levels to presence of metastases, in that 50% of patients with metastases had levels greater than 20 ng/ml. The average survival for the pathologic and normal category was almost similar, ranging from 13.27 to 16.81 months. The correlation between disease extent and survival was more sensitive for lactate dehydrogenase (LDH) than for CEA. So CEA as a tumor marker for SCLC must be applied in conjunction with other biomarkers, particularly LDH and neuron specific enolase (NSE) and is meaningful in only a small proportion of patients.