A 52-year-old Japanese woman was examined because of general malaise, weight loss and a lump in her left breast. She was diagnosed with cancer of the left breast and Graves' disease, and was administered methimazole (MMI). A left mastectomy was performed for the breast cancer. She presented with a high fever and peripheral blood examination revealed a severe pancytopenia. She was diagnosed with severe aplastic anemia, and administered G-CSF, however, the treatment was unsuccessful. Thus, oral methyprednisolone and cyclosporin were added. There was a remarkable improvement in the peripheral blood count.