Aims The severe form of coxa vara, the ‘shepherd's crook deformity’, is always a consequence of a locally extensive form of polyostotic fibrous dysplasia (or McCune-Albright syndrome). Treatment of this deformity is a challenge. The soft bone does not tolerate any implant that depends on the stability of the cortical bone (like plates or external fixators). Intramedullary nails are the most appropriate implants for stabilisation, but if they are inserted from the greater trochanter, they cannot correct the varus deformity enough. Patients and Methods We have developed a special intramedullary nail that can be inserted from the osteotomy site and can be driven retrograde into the femoral neck in an appropriate valgus position. We have operated 15 legs in 13 patients. The average age at surgery was 14 years and 5 months (6 to 28.9). In all, 11 femora had been operated before (unsuccessfully) with various implants. Results The average follow-up was 54.2 months (7 to 132). The average correction of the neck/(distal) shaft angle was 57.5° (10° to 80°) (= 72.8%). While pre-operatively none of the patients was able to walk without aid, at follow-up only one patient was unable to walk, three used the aid of crutches because of tibial lesions and one patient had an increased external rotation of the leg. At follow-up, most patients were free of pain. One implant broke and had to be replaced. Conclusion This new operative method offers the possibility of efficient correction and stabilisation of this severe and difficult deformation.