We present a case of pure red cell aplasia appearing four months after thymectomy for an invasive thymoma. A 61-year-old female whose chest X-ray demonstrated an anterior mediastinal mass was admitted to our hospital. Preoperative examination revealed neither anemia nor myasthenia gravis. During surgery, as it was found that the thymoma had invaded the upper left lobe of the lung and the left diaphragmatic nerve, these tissues were partially resected along with the thymus and the parathymic adipose tissue. Histologically the epithelial cells of the thymoma had round-oval nuclei with predominant lymphatic infiltration. There were some clusters of epithelial cells in the adjacent involuted thymic tissues. After postoperative radiotherapy of 40 Gy, the patient was discharged. About four months after the operation, she was readmitted because of anemia. Bone marrow aspiration demonstrated few erythroblasts but normal findings of granulopoiesis. Chest CT and radioisotopic examination revealed neither metastases nor recurrence of thymoma. Her anemia showed marked improvement by steroid therapy. After she was discharged, her condition has been uneventful in spite of tapering the steroid dose for seven months.