A 31-year-old female was well until few years ago when she was diagnosed as having Graves' disease. Methimazole (MMI) and Lugol's solution were prescribed. But 7 months later, she stopped taking them arbitrarily. Three months later, thirst and general fatigue appeared. Therefore insulin (60u/day) and MMI (30 mg/day) were administered and continued for 40 days. However no remarkable effect was brought about. She was then transferred to the radioisotope ward of Kumamoto Univ. Hospital and was treated with regular insulin only. Ten days later, she fell into thyroid storm associated with diabetic ketoacidosis and was transferred to our ward. We began to administer large volumes of transfusion, regular insulin, MMI, Lugol's solution, propranolol, hydrocortisone and digitalis. In 24 hours, ketoacidosis disappeared and she became alert. For hyperthyroidism, the dosage of MMI was increased to 60 approximately 45 mg/day and was continued for a month; however, her thyroid function did not normalize and agranulocytosis developed. MMI was discontinued, and she was treated with 131I. About a year later, she became euthyroid. Her diabetes mellitus was difficult to control during the hyperthyroid state but it was under good control with monocomponent lente insulin (36u/day) when the euthyroid state was resumed.