Hibernoma is a rare, benign soft tissue tumor composed of brown fat. A 42-year-old white man presented with an enlarging soft tissue mass located in his left medial thigh. This was evaluated radiographically by CT as well as MR imaging. An open biopsy revealed a hibernoma. The mass was subsequently excised in its entirety, maintaining a cuff of normal tissue. The patient has done very well postoperatively. Although nearly 100 cases have been reported, this case is presented to review the clinical and pathological presentation of this tumor and to describe its MR imaging characteristics.