Serum neuron-specific enolase (S-NSE) levels in 43 newly diagnosed untreated patients with small-cell lung cancer (SCLC) were compared with levels in 35 adult controls, 14 patients with non-small cell lung cancer (N-SCLC), and nine patients with noncancerous lung disease (N-CLD). The S-NSE level was raised (greater than or equal to 16 ng/ml) in 28 of 43 patients with SCLC, six of 16 patients with limited stage SCLC, and 22 of 27 of those with extensive stage SCLC. Extensive stage patients with SCLC had a significantly higher mean S-NSE level (50 ng/ml) than did limited stage patients with SCLC (16 ng/ml). Mean S-NSE levels in patients with N-SCLC and in patients with N-CLD were respectively 11 and 7 ng/ml. Serial measurements performed on 19 patients between the three-day-courses of chemotherapy showed an excellent correlation between S-NSE and clinical evolution. In addition, S-NSE was measured during the first three-day course of chemotherapy in 13 other patients; among them, seven had S-NSE levels greater than or equal to 100 ng/ml (mean: 490 ng/ml); these seven patients were responders; the remaining six had S-NSE levels less than 100 ng/ml (mean 28 ng/ml): among them, only two were responders. Such S-NSE measurements during the first cytostatic course seem to reflect the importance of tumor burden and may be valuable as early indicators of the response rate to chemotherapy.