We analyzed serum lactate dehydrogenase (LDH), neuron-specific enolase (NSE) and thymidine kinase (TK) levels in 22 patients with small cell lung cancer. Tumor proliferation was expressed as the proportion of S-phase cells (SPF), determined by DNA flow cytometry, from concomitantly taken biopsy samples. A positive correlation between serum NSE (r = 0.41) or LDH (r = 0.65, p = 0.05) levels and tumor SPF was noted, but was not found between serum TK levels and the SPF. The correlation between NSE and SPF was even more pronounced if only patients with extensive disease were considered (r = 0.77). The serum NSE and LDH, but not TK levels, were significantly greater in the patients with extensive disease (NSE 50.4 ng/ml, LDH 621 U/ml) compared to the patients with limited disease (NSE 21.0 ng/ml, LDH 272 U/ml, p = 0.05). Our results suggest that the combined determination of serum LDH and NSE levels gives valuable data on the primary tumor mass and its proliferative activity in small cell lung cancer.