症例は65歳男性，7年前に胸腺腫の摘除術を受けた後，毎年の定期健康診断で耐糖能は正常であった。1週間前から突如，口渇・多飲の代謝失調症状が出現し，高温多湿の職場で作業中に痙攣・意識障害で卒倒しているところを発見された。来院時の生化学検査では随時血糖1,846 mg/dL，HbA1c 6.7％，pH 7.15，K 9.2mEq/L，総ケトン体3,838μmol/Lと糖尿病性ケトアシドーシスを認め，持続インスリン静脈注射を含めた集学的治療により救命され，第20病日には強化インスリン療法で退院可能となった。一般的に急性発症または緩徐進行の1型糖尿病は他の自己免疫疾患を合併しやすく，胸腺腫との合併例が少数散見される一方，劇症1型糖尿病と胸腺腫の合併は本症例が世界で初めてであり，ここに報告する。

   A 65-year-old man had undergone resection of malignant thymoma 7 years ago, whose blood glucose levels had been maintained within a normal range after the resection. One week before the admission, he had subjective symptoms, such as thirst and polyuria. He suddenly fainted and was found to have collapsed with convulsions at workplace under hot and humid environmental conditions. Laboratory test showed diabetic ketoacidosis with hyperglycemia of 1,846 mg/dl and HbA1c levels at 6.7% accompanied with arterial pH of 7.15, potassium of 9.2 mEq/L, and total ketone bodies of 3,838 μmol/L. The patient was saved by intensive care with intravenous continuous insulin infusion. The patient was discharged from the hospital on day 20 with basal and bolus insulin therapy. We report here the first case of fulminant type 1 diabetes mellitus associated with malignant thymoma, suggesting the involvement of immunological derangement in the pathogenesis of this devastating disorder.