We report here the cases of two females with Graves' disease who developed insulin autoimmune syndrome after treatment with methimazole. The patients exhibited a sudden altered mental state after treatment with methimazole for approximately 4 weeks. Patients had hypoglycemia with serum glucose below 70 mg/dL, and laboratory findings showed both high levels of serum insulin and high titers of insulin autoantibodies. The two women had never been exposed to insulin or oral antidiabetic agents, and there was no evidence of insulinoma in imaging studies. After glucose loading, serum glucose, and total insulin levels increased abnormally. One of the patient was found to have HLA-DRB1*0406, which is known to be strongly associated with methimazole-induced insulin autoimmune syndrome. After discontinuation of methimazole, hypoglycemic events disappeared within 1 month. Insulin autoantibody titer and insulin levels decreased within 5 months and there was no further development of hypoglycemic events. We present these cases with a review of the relevant literature.