A total of 498 patients with small cell lung cancer received chemotherapy with or without chest irradiation at Osaka Prefectural Habikino Hospital from October 1977 through December 1991. Sixty-one who survived for more than two years were evaluated to determine the incidence and anatomic patterns of redevelopment of small cell lung cancer and development of second primary cancers. The numbers of expected cancers were estimated by cumulating person years of observation from 2 years after the start of treatment for small cell lung cancer to the date of death. Second primary cancers were observed in seven patients (four cases of non-small cell lung cancer, two of gastric cancer, and one of prostate cancer). The risk of a second primary cancer was 3.2 times greater than in the general population (95% Cl: 1.3-6.6). the relations between occurrence of a second primary cancer and family history of cancer, smoking history, smoking cessation after treatment of small cell lung cancer, and thoracic irradiation were studied. Occurrence of a second primary cancer correlated with family history (relative risk 7.5, 95% Cl: 1.5-22) and smoking cessation (relative risk 3.2, 95% Cl: 1.2-6.9). Long-term survivors were more likely to have a second primary cancer than a relapse of small cell lung cancer. Therefore, long-term survivors should be closely monitored for second primary cancers. Meta-analyses of studies done at several institutions may provide more detailed information on the occurrence of second primary cancers after small cell lung cancer.