Serum NSE levels were measured in 126 patients with previously untreated NSCLC. The NSE level was greater than 10 ng/ml in 29 patients (23.0%) and this was considered to be positive. Elevation of serum NSE levels correlated closely with clinical stage except stage I and II. The effect of chemotherapy was evaluated in 74 cases included 22 NSE-positive cases. The response rate was 50% in positive cases and 34.6% in negative cases. However, the median duration of response in positive cases (2.2 months) was significantly shorter than that in negative cases (6.6 months). The median survival time of 6.0 months in positive cases was brief compared with 9.6 months in negative cases. These results indicate that elevation of serum NSE level in patients with NSCLC may be a marker of poor prognosis.