A method and a system for determining an angle between two parts of a bone that are twisted relative to one another about the axis of a bone shaft. The system is particularly suitable for determining the antetorsion angle of a femur. In order to determine an antetorsion angle of a bone easily intraoperatively, a method with the following steps is proposed: establishing the position of a first orientation feature assigned to a first part of the bone, in particular a femoral neck axis, using an imaging method, establishing the position of a second orientation feature assigned to a second part of the bone, in particular a condyle tangent or a condyle plane, using an imaging method, and determining the angle, in particular the femoral antetorsion angle, from the positions of the two orientation features.