In order to avoid the use of a marking element on the pelvic bone in a method for determining the mechanical axis of a femur, with which the femur is moved about the hip joint, the movement of the femur is followed via a navigation system by means of a marking element on the femur, position data of the femur obtained therefrom are stored and the position of the mechanical axis of the femur is calculated relative to the same from the various position data of the femur in various positions, it is suggested that the femur be pivoted from an initial position only through a maximum pivoting angle of 15.degree. in various directions and that the mechanical axis of the femur be calculated from the position data of the surface area thereby covered by the marking element and from the position data of the knee joint otherwise determined. In addition, a device for carrying out this method is described.