The first patient was a 37-year-old man with an invasive and lymphoid cell dominant thymoma (stage III). He underwent extended total thymectomy and partial resection of the upper lobe of the left lung. Four years after the operation, he had ptosis and diplopia and was diagnosed as having myasthenia gravis (positive Tensilon test and raised antiacetylcholine receptor antibody titer). His symptoms improved with the steroid therapy. The second patient was a 37-year-old woman with an invasive and mixed type thymoma (stage III). Extended total thymectomy with combined resection of the mediastinal pleura and right phrenic nerve was performed, but the tumor recurred in the right thorax 2 years postoperatively. Subtotal resection of the parietal pleura and recurrent tumors was performed by right thoracotomy, and steroid therapy was given. She developed malaise, ptosis and diplopia three months later, and was diagnosed as having myasthenia gravis. Her symptoms disappeared after the steroid therapy was stopped. A review of the Japanese literature is presented and problems regarding the pathogenesis of this disease are discussed.