A 30-year-old female complained of lancinating pain in the bilateral thighs for 10 days. The patient had a 22-year history of insulin-dependent diabetes mellitus. Physical examination revealed swelling of the bilateral lower extremities. There was exquisite tenderness on palpation over the medial thighs, with marked increase in pain on hip and knee flexion. Muscle strength of quadriceps, hamstrings, and hip adductor was decreased due to muscle pain. Pedal pulses were palpable bilaterally. Roentogenograms of the left femur revealed calcification of the left femoral arterial wall. Venogram revealed no obstruction with normal drainage. Complete blood cell count showed left shift of the neutrophils, markedly accelerated erythrocyte sedimentation rate, prolonged prothorombin time of 9 sec (normal 11.7 sec), C-reactive protein of 7.3 mg/dl and serum creatine kinase level of 175 IU/L. FBS was 225 mg/dl and Hb A 1 c was 16.4%. An MR imaging of the thighs revealed high signal intensities in the bilateral adductor muscles on T 2-weighted images. The symptoms resolved spontaneously over a three week period. From the course of the illness and MR imaging, the patient was diagnosed having diabetic muscle infarction (DMI), a rare complication of diabetes mellitus. To our knowledge, this is the first reported case of DMI in Japan. Diabetic microangiopathy and hypercoagulability are thought to be responsible for inducing DMI. Because the diagnosis can be made from the characteristic clinical and the typical MR imaging findings, muscle biopsy is not always necessary to obtain the diagnosis of DMI.