Pure red cell aplasia is unusual cause of anemia and a selective aplastic disorder that affects the erythroid series of the bone marrow. Fifty percent of all patients with red cell aplasia will have a thymoma. Twenty-five to 30% of those who undergo thymectomy will be cured. A 57-years-old man was admitted to the medical department of Korea University hospital with complaints. Physical examination reveals a sick looking man with a pale lip, anemic conjunctiva and subicteric sclera. On auscultation, coarse breathing sound and moist rale was heard on the right lung field. Neither the liver nor spleen was palpable. A blood count showed the erythrocytes to number 2,640,000/mm3 and hemoglobin to be 7.0gm/dl. A white blood cell count was 5,000/mm3 and a platelet count was 328,000/mm3 Reticulocyte count was 0.7%. Examination of the peripheral blood smear showed the red cell, to be normocytic and normochromic. Urine sugar was three positive and GTT was positive. The anterior-posterior and lateral view of Chest X-ray was suggestive of an anterior mediastinal mass. A bone marrow biopsy reveals absence of red cell precursors and a normal myeloid series and megakaryocytes. At thoracotomy in May 1980 an encapsulated, lobulated, benign thymoma, which measured 5x7x5 cm was removed, microscopic examination showed it was of the spindle cell type. The postoperative course was uneventful, but the patient never had a return of hemoglobin to the blood. The patient was discharged on the postoperative] 3 days. At postoperative 1 month, the patient was readmitted for bone marrow study and had no return of red cells to bone marrow. At now, patient has been treated with steroid and the further follow up study will be needed.