Xanthoma is a lesion containing abundant foamy histiocytes most commonly occurring in superficial soft tissues such as skin, subcutis, or tendon sheaths. The involvement of deep skeletal structures, however, is rare and has only been infrequently reported in the English literature. Most xanthomas occur in patients with hyperlipidemic disorders. We report a case of a xanthoma in the sacrum and ilium of a patient with hyperlipidemia type IIa, who had chronic lower back pain for more than 20 years. On radiographs the lesion appeared multiloculated and osteolytic with a thin sclerotic border and containing multiple nodular calcifications within its matrix. Computed tomographic images revealed a presacral soft-tissue mass that also infiltrated the adjacent sacroiliac joint and iliac fossa. On histologic examination, abundant areas of xanthoma cells and cholesterol clefts, typical of xanthoma, were present. The patient received simple curettage of the lesion, and his symptoms were markedly relieved.