We retrospectively reviewed the records of 11 thymoma patients to clarify the role of hemithoracic irradiation and chemotherapy for patients with thoracic dissemination at the initial presentation. Radiotherapy was administered postoperatively in all but two patients who were not candidates for surgical treatment. Radiotherapy doses ranged from 10 to 17 Gy for the entire hemithorax of the disseminated site and from 30 to 55 Gy to the primary tumor bed. Chemotherapy of various protocols was also employed preceding radiotherapy in seven cases. Cumulative 5- and 10-year survival rates were 80% and 64%, respectively. Regrowth-free 5- and 10-year survival rates were 55% and 37%, respectively. Six of the 11 patients were free from regrowth at a median follow-up interval of 60 months. The role of chemotherapy remained unclear. Acute or late sequelae were modest and acceptable. In conclusion, entire hemithoracic irradiation is justified as postoperative or definitive radiotherapy treatment for disseminated thymoma unless there is a risk of radiation pneumonitis. Further evaluation should be conducted.