Background: The purpose of this study was identify prognostic factors and to investigate the association between postoperative radiotherapy and overall survival of thoracic esophageal squamous cell carcinoma patients. Methods: From January 1993 to March 2007, 1,715 patients underwent extended esophagectomy with three-field lymph node dissection with or without postoperative radiotherapy and were eligible for analysis. Patients were grouped to surgery only (n = 1,277) and surgery plus postoperative radiotherapy (n = 438). Radiation dose was 50 Gy in 25 fractions. Results: The overall survival rates at 1, 3, 5, and 10 years were 86.6%, 61.3%, 49.4%, and 36.1%, respectively. Univariate and multivariate analyses showed that age 60 years or more, male sex, tumor more than 5 cm long, poorly differentiated histology, T4 tumor, presence of a vascular cancer thrombus in the surgical specimen, lymph node positivity, 3 or more positive lymph nodes, and disease stage II or higher were negative prognostic factors for overall survival. Postoperative radiation therapy improved overall survival for patients with poor disease-related prognostic factors: positive nodal disease, 3 or more positive lymph nodes, stage III/IV, and large or deeply invading tumor. Postoperative radiation had no survival benefit for patients who did not have the poor disease-related prognostic factors. Conclusions: Postoperative radiotherapy is indicated for patients with poor disease-related prognostic factors. © 2010 The Society of Thoracic Surgeons.