A trigonometric analysis of the effect of a proximal femoral osteotomy, either dome or cuneiform, on leg length was done to aid the preoperative planning for this surgery. Change in leg length may be determined for a dome osteotomy by multiplying the length of the head-neck segment by the difference between the cosines of the preoperative and postoperative neck-shaft angles as measured from the vertical. A series of determinations are tabulated for a variety of head-neck lengths and angulation changes. When a cuneiform osteotomy is being done, the effect of the wedge can be determined and should be considered. Also to be considered are a tendency for spontaneous correction and growth stimulation from the osteotomy.