Purpose: This analysis was performed to determine the most appropriate volume that should be encompassed by thoracic radiation treatments (TRTs) for patients with limited-stage small-cell lung cancer (LSSCLC) who have responded to initial chemotherapy. Patients and Methods: A retrospective review of all patients (N = 67) with LSSCLC who were not entered onto a research protocol and were treated at our institution between the years of 1982 and 1990 was performed. Fifty-nine of 67 patients had adequate information regarding the size of the tumor before the start of chemotherapy (computed tomographic [CT] scan of chest or chest x-ray), the size of the tumor before TRT, and the TRT field size based on a simulation radiograph. All 59 patients were treated with cyclophosphamide-based chemotherapy, and TRT was generally delivered concomitantly with chemotherapy following two to three cycles of chemotherapy alone. Results: Of 59 patients, 28 were treated with TRT field sizes that encompassed postchemotherapy tumor volumes, and 31 patients were treated with TRT field sizes that encompassed prechemotherapy tumor volumes (defined as a volume that included at least a 1.5-cm margin on the prechemotherapy tumor volume). Nineteen patients had an intrathoracic recurrence of disease as the first site of recurrent small-cell carcinoma: 10 of 31 patients treated with TRT fields that encompassed prechemotherapy tumor volumes and nine of 28 patients treated with TRT fields that encompassed postchemotherapy tumor volumes. For the 28 patients treated with TRT fields that encompassed postchemotherapy tumor volumes, the greatest distance that the prechemotherapy tumor volume (without margins) extended beyond the edge of the TRT field was 0.5 to 5.0 cm, with a median of 2.5 cm. All 19 of the intrathoracic recurrences were in-field failures, although two patients (one prechemotherapy volume and one postchemotherapy volume) did have concurrent pleural effusions. Conclusion: These results indicate that the use of TRT fields that encompass postchemotherapy tumor volumes does not increase the risk of marginal failures or intrathoracic failures outside the TRT field.