Serum neuron-specific enolase (NSE) was measured in 48 newly diagnosed untreated patients with small cell lung cancer (SCLC) by radioimmunoassay. Serum NSE level elevated (greater than or equal to 15 ng/ml) in 50% of all patients. The positive ratio of NSE in patients with extensive disease (64%) was significantly higher than those in the patients with limited disease (30%) (p less than 0.05). The positive ratio of NSE in the patients with one metastatic site was 50%, that with two or more metastatic sites was 100% (p less than 0.05). No significant correlation was found between serum NSE levels and metastatic site as well as between serum NSE levels and response to the chemotherapy. In the patients with extensive disease, survival time was shorter in the patients with positive NSE levels than the patients with normal NSE levels. These findings indicate that serum NSE may be a useful marker for staging, monitoring and prognosis in patients with SCLC.