Erdheim-Chester disease is a distinctive pathologic and radiographic entity characterized by bilateral symmetric sclerosis of the diametaphyseal regions of long bones and infiltration of foamy lipid-laden histiocytes. It is a rare histiocytic disease of unknown etiology that is characterized pathologically by xanthogranulomatous infiltrates of multiple organs. We present a patient in her early sixties with bilateral mild knee and leg pain. The patient showed a typical bilateral symmetric medullary sclerosis at the diametaphyseal portions of long bones of the lower extremity. The diagnosis was confirmed by a bone biopsy, and bisphosphonate (alendronate, 70mg/wk) was given to the patient. After 9 months of treatment, biochemical markets of bone turnover, which were high at baseline, decreased to normal ranges. However, the radiographs showed that bone lesions had changed to lytic lesions. We propose use of bisphosphonates, such as alendronate, to decrease the biochemical markers of bone turnover. But we suggest that it is premature to conclude that bisphosphonates have any effect on lytic lesions and the progression of the disease as shown by changes in radiographs. Further studies with long-term follow-up and ultrastructural evaluation are needed. (c) 2005 by American Congress of Rehabilitation Medicine and the American Academy of Physical Medicine and Rehabilitation.