We report on a patient with an unusual, slowly enlarging hematoma of the left buttock. A 62-year-old man presented with a 6-year history of an enlarging mass of the left buttock. He had first noted the mass 6 years earlier and had had sciatica of the left lower limb for the last 2 months. He denied any history of antecedent trauma. The lesion extended over 3 gluteal muscles (the gluteus medius, gluteus minimus, and piriformis). On microscopic examination, the lesion showed typical signs of chronic expanding hematoma. The sciatica was relieved after surgical removal of the lesion. The lesion had not recurred at the last follow-up 4 years after the operation. The present case suggested that chronic expanding hematoma can extend into multiple muscles due, perhaps, to long-term growth and the anatomical and functional conditions of the affected muscles. Our case also suggests that chronic expanding hematoma can be a cause of piriformis syndrome.