The authors have made an anatomical study on cadavers to determine more precisely the mechanical goal of Chiari osteotomy and its achievement. They note that the procedure aims to improve the lateral and anterior coverage of the femoral head, to suppress subluxating forces and to medialize the hip to decrease the tension of the gluteus medius. The line of osteotomy must allow rotation of the lower fragment of the iliac bone beneath the upper part, the axis of rotation being the symphysis pubis. The ideal line of osteotomy should be conical and as vertical as possible but leaving the sacro iliac joint unimpaired. Its achievement is made easier by a pre-operative drawing in which the following points should be respected. The sagittal displacement should be calculated on a true antero posterior radiograph in which the direction of the axis of the pubic symphysis can be appreciated. The correct position of the guide pins should be determined by the location of its point of entry and exit and not by its obliquity which may be misleading. Medialization should be obtained by abduction of the thigh and not by pressure on the greater trochanter to avoid total pelvic translation. Thirty-eight patients have been operated on and followed up for more than one year. Twenty one results were satisfactory.