Serum levels of NSE were monitored in 20 SCLC patients who completely responded to combination chemotherapy. An elevation of NSE was observed in five of nine patients with recurrent disease, but predated a relapse in only one. No elevation in NSE level was noted in nine patients who remained in complete remission. The addition of serial assays of LDH to NSE monitoring did not result in any gain in early warning of relapse. This study suggests that the value of serial NSE measurements for predicting a relapse of disease is limited.