Contents We describe herein an unusual case of simultaneous presentation of a fulminant type 1 diabetes and a Graves’ disease. A 67-year old male was admitted to the hospital with nausea, vomiting, and abdominal discomfort. He had a palpitation, hand tremor and mild weight loss during last month and his conditions aggravated acutely with nausea and vomiting, prior 3 days before admission. Laboratory findings included: glucosuria(+++), ketonuria(+++), serum glucose 677 mg/dL, HbA1c 6.5%, insulin 0.614 uU/mL, c-peptide 0.223 ng/mL, amylase 100 IU/L, lipase 259 U/L, GAD antibody 0.74 [0-0.9] U/mL, IA-2 antibody < 0.4 [0-0.4] U/mL, insulin antibody 5.4 [0-7]%, free T4 3.33 [0.93-1.7] ng/dL, TSH 0.007[0.27-4.2] uIU/mL, TSH receptor antibody 4.83 [~1.75] IU/L. An abdominal CT scan was showed normal pancreas, and a Tc-99m pertechnetate thyroid scan revealed homogeneously increased activity in the thyroid gland (20 minute Tc-pertechnetate uptake = 52.5). The clinical characteristics of the patient were remarkably abrupt onset diabetes, very short duration of diabetic symptoms, acidosis at the time of diagnosis, negative findings for islet-related autoantibodies and no c-peptide secretion. His diagnosis was compatible with fulminant type 1 diabetes. And, thyroid function test with autoantibody and thyroid scan results were suggestive Graves’ hyperthyroidism. We often observed classic type 1A diabetes associated with autoimmune thyroid disease. This case was unique in that fulminant type 1 diabetes developed simultaneously with Graves’ hyperthyroidism