Aplastic anemia is a rare but severe complication of methimazole (MMI) treatment for Graves' disease. We present a case of a 53-year-old Japanese female who had been treated with 30 mg/d of MMI for 30 days for Graves' disease and was subsequently admitted to the Japan Self Defense Forces (JSDF) Central Hospital with a mild sore throat and high-grade fever that began the previous day. The patient had a reduced white blood cell count (WBC) count of 0.9 x 10(3) per microliter with severe granulocytopenia and increased lymphocytes, a platelet count of 49 x 10(3) per microliter, and hemoglobin of 10.6 g/dL. Bone marrow (BM) aspirates showed hypocellular bone marrow with plasmacytosis. Because of poor recovery of her peripheral blood values after withdrawal of MMI, she was given transfusions of platelets and erythrocytes thereafter. This is the second report of plasmacytosis in bone marrow of MMI-induced aplastic anemia, and suggests that immunogenic mechanisms may cause this rare complication.