The outcome of eight patients with invasive thymoma accompanying pleural dissemination was investigated. Only two patients had mediastinal tumor resection and pleural disseminated tumor excision. Seven patients underwent radiotherapy to the mediastinum and/or disseminated tumors. A clinical response to radiotherapy was achieved in the six patients with evaluable lesions (complete response in five patients and partial response in one). The estimated 5-year survival rate was 87.5%. Four patients were alive more than 10 years. So far, the mediastinal tumors of seven patients have been controlled for periods ranging from 42 to 154 months. Recurrence in six patients appeared as pleural tumors. Four out of the six patients had five courses of radiotherapy to the recurrent pleural tumors, four of which achieved complete response. No distant metastases were observed at any time. These observations suggest that radiotherapy should be the primary mode of treatment in cases of invasive thymoma with pleural dissemination.