Intraosseous lipomas are rare bone tumors, the pathogenesis of which remains controversial. In the involution of lipomas,many conditions were reported, such as fat necrosis, cystic formation, and dystrophic calcifi cation. Moreover, degenerationof an intraosseous lipoma into an aneurysmal bone cyst (ABC) is extremely rare. We report a patient who had coexistingfeatures of an intraosseous lipoma and ABC diagnosed by a radiologist, pathologist, and orthopedic surgeon. Nosuch case has been published, and we propose a possible mechanism according to the most tenable hypothesis. The patientwas successfully treated with intralesional curettage and implantation of polymethylmethacrylate. There are a numberof hypotheses regarding the etiologic factors for intraosseous lipomas and ABCs. The rare case reported herein mayprovide some clues about the pathogenesis of these two diseases.