The level of serum neuron-specific enolase (NSE) has been implicated as a prognostic factor for patients with small cell lung cancer (SCLC). A prospective evaluation was undertaken to assess the prognostic significance of pretreatment NSE and treatment-induced minimum NSE values in patients with SCLC. Patients from two Phase III North Central Cancer Treatment Group trials [one for patients with extensive stage SCLC and one for patients with limited stage SCLC] were asked to enter this laboratory correlational trial. Both trials included treatment with four to six cycles of etoposide and cisplatin, and 121 patients (71 extensive stage SCLC and 50 limited stage SCLC) were entered into the present study of NSE. Pretreatment NSE values and treatment- induced minimum NSE values were independent predictors of time to progression and survival in multivariate analysis. Hazard rate modeling allowed the formulation of specific relationships of NSE to time to progression and survival. Pretreatment NSE levels inversely correlated with time to progression and survival in these patients with SCLC. Pretreatment NSE accounted for 28% of the variance in survival. Both pretreatment NSE and treatment-induced minimum NSE were independent prognostic predictors of time to progression and survival.