Histiocytosis X can exhibit a variety of musculoskeletal manifestations. Nonetheless, this case exhibits two striking features. Severe progressive scoliosis and massive osteolysis of both femoral shafts occurred in conjunction with the Hand-Schüller-Christian form of histiocytosis X. The scoliosis was treated successfully by posterior fusion and instrumentation. The clinical features of histiocytosis X are reviewed. Speculation as to the etiology of these features is presented.