A 44-year-old woman with a 5-year history of poorly controlled Type 1 diabetes mellitus presented with a painful, firm and warm swelling in her right thigh. Pain was severe but the patient was not febrile, and had no history of trauma or abnormal exercise. Laboratory tests showed ketoacidosis, major inflammation (erythrocyte sedimentation rate (ESR) = 83 mm/h), normal white blood cell count and normal creatine kinase level. Plain radiographs were normal, and there were no signs of thrombophlebitis at Doppler ultrasound. Magnetic resonance imaging (MRI) showed diffuse enlargement and an oedematous pattern of the adductors, vastus medialis, vastus intermedius and sartorius of the right thigh. The patient's symptoms improved dramatically, making biopsy unnecessary, and a diagnosis of diabetic muscular infarction was reached. Idiopathic muscular infarction is a rare and specific complication of diabetes mellitus, typically presenting as a severely painful mass in a lower limb, with high ESR. The diabetes involved is generally poorly controlled longstanding Type 1 diabetes with established microangiopathy. Differential diagnoses include deep vein thrombosis, acute exertional compartment syndrome, muscle rupture, soft tissue abscess, haematoma, sarcoma, inflammatory or calcifying myositis and pyomyositis. In fact, physician awareness should allow early diagnosis on the basis of clinical presentation, routine laboratory tests and MRI, thereby avoiding biopsy and its potential complications as well as unnecessary investigations. Rest, symptomatic pain relief and adequate control of diabetes usually ensure progressive total recovery within a few weeks. Recurrences may occur in the same or contralateral limb.