Relative abductor strength (RAS) is defined as the difference between the available (maximum) abductor force and the required abductor force i.e. the force needed to maintain the mechanical equilibrium of the pelvis. In this work the dependence of RAS on the centrum-collum-diaphyseal angle and on the position of the greater trochanter was studied using a 3-D mathematical model of the human hip in the monopodal stance. Simulation of the varus and valgus osteotomy was done for the normal, 3 cm distal and 5 cm proximal position of the greater trochanter before the operation. It was shown that after valgi-sation a significant reduction in RAS is to be expected. On the other hand, varisation osteotomy is not associated with a significant change in RAS. The changes in RAS after valgisation for normal and distal position of the greater trochanter occur within the scope of the negative Trendelenburg sign. A negative RAS and, consequently, a positive Trendelenburg's sign occur only in extreme valgisation with a proximally situated greater trochanter. Reduction in RAS may reduce the capability of the hip muscles during strenuous physical activity and may thus participate to earlier muscle fatigue. The importance of special rehabilitation programme after valgisation osteotomy is stressed. Key words: hip biomechanics, abductor muscles, varus and valgus osteotomies, proximal femur.