Chronic expanding hematoma (CEH) is a rare type of hematoma that expands slowly and continuously without compromising coagulation. Its etiology is often unknown. However, we experienced a rare case of CEH, in which an epidermal cyst was thought to be the cause. A 57-year-old man had developed a painless soft-tissue tumor in his left buttock 45 years earlier, which slowly grew to 11 cm in diameter. Preoperative magnetic resonance imaging suggested a large cyst containing several masses. Surgery revealed a hematoma and keratin debris surrounded by a white fibrous cyst wall and a fibrous nodule measuring 4 cm in diameter. On histopathological examination of the white fibrous wall, an epidermal component was observed contralateral to the superficial punctum, but the epidermal component was absent from most of the wall, including the fibrous nodule. Based on a pathological examination, the CEH was suggested to have been caused by partial rupturing and inflammation of an epidermal cyst. To the best of our knowledge, there are no reports of epidermal cysts causing CEH. In addition, the large fibrous nodule protruding from the CEH cyst wall was considered to be rare. This was considered to be a rare CEH that may have originated from an epidermal cyst.
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