PURPOSE: Retrospective data suggests prolonging the time to complete thoracic radiotherapy (TRT) may negatively impact tumor control and survival in limited stage small cell lung cancer (LSCLC). We examined the association between TRT duration and outcomes on a prospective phase III study. MATERIAL AND METHODS: This review included 267 patients who received protocol TRT on a phase III CALGB LSCLC study assessing the addition of tamoxifen to standard chemo-radiotherapy. TRT, to a planned dose of 50Gy in 2Gy daily fractions, was initiated with the fourth chemotherapy cycle. TRT interruptions were mandated for hematologic toxicity (granulocytes<1000/mm3 or platelets<75,000/mm3) and esophageal toxicity (dysphagia necessitating intravenous hydration). RESULTS: TRT interruptions > or =3 days occurred in 115 patients (43%), most frequently during the 4th week of TRT, and did not differ between treatment arms. Hematologic toxicity and esophageal toxicity were the most frequent indications for interrupting TRT. Variables including advanced age ( > 70 years), gender, race, or radiotherapy treatment volume did not predict for TRT interruptions. Overall survival (OS) and local tumor control did not correlate with the administration of TRT interruptions or with TRT duration. CONCLUSION: Toxicity mandated interruptions of conventional dose, once-daily, TRT may not adversely affect outcomes for patients receiving TRT concurrent with chemotherapy (cycle 4) for LSCLC. The implications for accelerated or high dose TRT regimens are not clear.