A 63-year-old woman was admitted to a neighborhood hospital because of difficulty in holding her head up in December, 1988. Medical therapy for myasthenia gravis was started. A simple X-ray film and CT of the chest revealed a mediastinal tumor. An enlargement of the tumor and severe anemia developed, then pure red cell aplasia was diagnosed in April, 1992. The patient was admitted to the hospital for a surgery of the mediastinal tumor. On admission, the classification of her myasthenia gravis was Osserman II a type. The activity was eased with an anti Ch-E drug. CT revealed that the mediastinal tumor was egg-sized and located in the fore-mediastinum with internal calcification, and its invasion into the left lung was lightly suspected. At the time of the operation a thymoma located in the lower left lobe of the thymus invading the left pleura and left lung was observed, so that the extended thymectomy including this site was performed. Histological diagnosis of the tumor revealed malignant invasive thymoma. After the operation, radiotherapy was added. We here report this rare case of autoimmune disease with a malignant thymoma, and survey of the literature on this subject is added.