The pattern of relapse was analyzed in patients with small cell lung cancer (SCLC). Of 180 patients treated with intensive combination chemotherapy between 1976 and 1987, 75 achieved complete response (CR). Of 47 patients with limited disease (LD), 20 (43%) initially relapsed in the chest and 7 (15%) in the brain. Among 27 patients with extensive disease (ED), the chest was also the most frequent site of relapse (44%) followed by the brain (19%). In LD patients who had received chemotherapy plus chest irradiation, the initial relapse rate and the cumulative relapse rate in the chest at 2 years were only 29% and 35.4%, respectively. These rates were significantly lower compared with the rates of 69% and 76.5% for patients who had received chemotherapy alone (p less than 0.05). Survival was improved to some extent by the addition of chest irradiation, but not significantly, however, the long-term survival rate favored those receiving chest irradiation. Prophylactic cranial irradiation (PCI) reduced the frequency of brain relapse and significantly improved the survival of SCLC patients achieving CR. The median survival time and 5-year survival rate of patients who received PCI were 23.1 months and 26.7%, which these figures were only 14.0 months and 8.3% for those who had not respectively. Analysis using Cox's proportional hazard model showed that PCI was the greatest prognostic factor favoring the SCLC patients achieving CR. These results indicate that chest irradiation and PCI in conjunction with intensive combination chemotherapy are effective for cases of SCLC with CR.