Serum samples were collected from 115 small-cell lung cancer patients before each course of chemotherapy and during follow-up. Levels of neuron-specific enolase (NSE) were measured and compared to the clinical assessments of the course of the disease, which were done by the responsible physician without knowledge of NSE-values. The predictive accuracy of an increase or decrease of NSE for a major response (CR + PR), SD or PD was 98%. Importantly no false-positive rises of NSE were observed. On the basis of this large number of data it seems justified to conclude that in common clinical practice the treatment of small-cell lung cancer patients can be monitored by serial measurements of NSE alone.