To evaluate the optimal timing for thoracic radiotherapy (TRT) in limited-stage small-cell lung cancer (LS-SCLC), the Lung Cancer Study Group of the Japan Clinical Oncology Group conducted a phase III study in which patients were randomized to sequential TRT or concurrent TRT. We treated 231 patients with LS-SCLC. TRT consisted of 45 Gy over 3 weeks (1.5 Gy twice daily), and the patients were randomly assigned to receive either sequential or concurrent TRT. All patients received four cycles of cisplatin plus etoposide every 3 weeks (sequential arm) or 4 weeks (concurrent arm). TRT was begun on day 2 of the first cycle of chemotherapy in the concurrent arm and after the fourth cycle in the sequential arm. Concurrent radiotherapy yielded better survival than sequential radiotherapy (P =.097 by log-rank test). The median survival time was 19.7 months in the sequential arm versus 27.2 months in the concurrent arm. The 2-, 3-, and 5-year survival rates for patients who received sequential radiotherapy were 35.1%, 20.2%, and 18.3%, respectively, as opposed to 54.4%, 29.8% and 23.7%, respectively, for the patients who received concurrent radiotherapy. Hematologic toxicity was more severe in the concurrent arm. However, severe esophagitis was infrequent in both arms, occurring in 9% of the patients in the concurrent arm and 4% in the sequential arm. This study strongly suggests that cisplatin plus etoposide and concurrent radiotherapy is more effective for the treatment of LS-SCLC than cisplatin plus etoposide and sequential radiotherapy.