Malignancy spread throughout a hemithorax without distant metastasis poses a difficult therapeutic challenge. Irradiation is often not considered because of the risk of pulmonary and cardiac toxicity. We report on a patient with thymoma recurrent throughout the left pleural cavity. Disease progressed despite chemotherapy, and subsequently a radical course of radiotherapy (6,600 cGy) was delivered to the entire hemithorax. Tumor regressed markedly by the completion of radiotherapy. Although tumor regrowth was noted 1 year after radiotherapy, the patient remained markedly improved symptomatically until shortly before her death 2 years after radiotherapy. Pulmonary function tests at 1 year (forced expiratory volume of the first second and forced vital capacity) were similar to pretreatment values, and cardiac function at 2 years remained essentially normal. High-dose hemithorax irradiation may be a consideration in select cases.