The authors report on 21 cases of "primary" xanthoma of bone. Twenty of the patients were older than 20 years old. The male-female ratio was 2:1. The presenting symptom was pain in 13 patients and neurologic symptoms in 2; in 6 patients, the lesion was an incidental finding. All but one of the lesions in this series were solitary, and the flat bones (pelvis, rib, skull) were the most frequently involved sites. Radiographically, a well-defined, sometimes expansile lytic lesion, with either a small area of surrounding reactive bone or a distinct sclerotic margin, was seen. Microscopically, foam cells, giant cells, cholesterol clefts, and fibrosis were present in varying degrees. In none of these cases was there an identifiable underlying lesion. The differential diagnosis includes Erdheim-Chester disease (a multisystemic granulomatosis) and bone involvement in sinus histiocytosis with massive lymphadenopathy (Rosai-Dorfman disease). More important is the differential diagnosis with metastatic clear cell carcinoma. Xanthoma of bone is a benign lesion, and complete or even partial removal is effective. Xanthomas may represent a "burnt-out" benign condition such as fibrous dysplasia or histiocytosis X.