In approximately 25% of patients with hereditary multiple exostosis, there is an abnormal osteochondral formation localized in the femoral proximal metaphysis. This formation often causes a mechanically progressive insufficiency of the acetabular cavity, a true developmental hip dysplasia, that together with a coxa valga deformity, which is also present, causes a gradual deterioration in the relations of this joint. This malformation has a poor prognosis and is difficult to manage. Although this entity is rather frequent and quite