Chronic expanding hematoma (CEH) is a hematoma that persists for > 1 month and gradually increases after trauma or surgery. Herein, we present a case of a 73-year-old man with CEH in his right thigh. He had sustained a contusion in a traffic accident approximately 10 years before. Two years after the accident, two subcutaneous masses were discovered in his proximal right thigh. Eight years later, another subcutaneous mass was observed in the distal part of the same thigh. All three masses were surgically removed, and CEH was diagnosed based on the clinical course, imaging, and pathological examination. The first two masses were thought to be related to the traffic accident. The cause of the third mass was uncertain; however, minor trauma and anticoagulant medication may have contributed to its development. A detailed history is important to determine the cause of CEH development.
