Three hundred ninety patients who underwent thymectomy for myastheniagravis (MG) were followed up to investigate the development of associatedmalignancies. There were 102 patients with thymoma and 288 without thymoma.Malignant neoplasms were detected in ten patients, four of whom already hadthe tumor at the time MG was diagnosed. Thus, malignancy developed afterthymectomy in six patients. Malignant fibrous histiocytoma (MFH) developedin three patients, as well as gastric cancer, gastric leiomyosarcoma,rectal cancer, liver cancer, lung cancer, breast cancer, and thymiccarcinoid in one patient each. Nine of the ten malignancies developed inthe thymoma group, and only one in the non-thymoma group. The predictednumber of patients with malignancy was 2.63 in the thymoma group and 2.65in the non-thymoma group. Our findings suggest that the presence of thymomafacilitates the occurrence of extrathymic malignancy, and that thymectomynever enhances the occurrence of malignancy but possibly inhibits it.