A positioning aid 1 has a coupling element 7 for coupling to the prosthetic acetabular component, as well as an auxiliary body 9 that is fixed relative to the coupling element and which is provided with recesses. The center lines of said recesses are at angles relative to each other, in one of the recesses a ball compass 13 is inserted. This recess corresponds to the desired orientation in which the acetabular prosthetic component 3 is to be mounted in the acetabulum 5. Inscriptions are provided on the auxiliary body near each recess indicating two angles at which the recess is relative to two vertical reference planes which are perpendicular to each other. When the auxiliary body 9 is kept level, the angles indicated near the recess in which the circular bubble spirit level compass 13 is inserted defines the position in which the acetabular prosthetic component is oriented.