Hematomas following surgery or trauma usually resolve without complications. In some instances, for reasons that are not completely understood, hematoma formation is followed by slow expansion of the mass. The large fluid collection is surrounded by a pseudocapsule and fibrous tissue, and present months or years after the original insult, as chronic expanding hematoma. In this case study, we present a rare case of chronic expanding hematoma in an 84-year-old woman. The patient noted a painful swelling in the posteromedial aspect of the lower thigh, and recalled a bump to the left knee 64years earlier. The lesion was successfully resected, with a preoperative presumptive diagnosis of organized hematoma, although the clinical findings were suggestive of malignancy. We describe the radiological findings, discuss the differential diagnosis.