Carcinoembryonic antigen (CEA), neuron-specific enolase (NSE), and the BB isoenzyme of creatine kinase (CK-BB) were evaluated before therapy in the sera of 195 patients with histologically confirmed small cell lung cancer (SCLC) in a prospective multicenter trial. Forty-four percent (84 of 193) of all patients had CEA levels higher than 5 ng/ml, 66% (111 of 168) had NSE levels higher than 12.5 ng/ml, and 32% (40 of 123) had CK-BB levels higher than 10 ng/ml. Clear pathologic levels were less frequently observed. Significantly higher pretreatment titers for CEA, NSE, and CK-BB were found in patients with bone marrow and/or liver metastases. The most elevated marker levels were observed in the group of nonresponding patients with bone marrow and/or liver metastases. Only a slight correlation between the pretreatment CEA level and survival time could be observed. Patients with pathologic NSE (greater than or equal to 30 ng/ml) levels and, in particular, those with pathologic CK-BB (greater than or equal to 25 ng/ml) levels had a significantly shorter median survival than those with normal or elevated levels. In addition, a positive linear correlation between pretreatment NSE and CK-BB (n = 116, r = 0.54) levels was found, but CEA levels did not correlate with other marker levels. From these data it is concluded that pretreatment CEA, NSE, and CK-BB levels are helpful in the clinical management of a subset of patients with SCLC, i.e., those with bone marrow and/or liver metastases.