An acetabulum sizer and drill guide for use in implanting an acetabular cup prosthesis comprises a substantially hemispherical shell having an outside surface that generally conforms to the outside surface of the prosthesis to be fitted into a prepared acetabulum. Viewing ports in the shell enable visual inspection of the acetabulum during sizing thereof. A circular peripheral rim on the shell is generally flush with the peripheral boundary of a properly sized acetabulum when the shell is seated therein. Drill guide structure on the shell functions to guide a drill bit into the pelvis bone adjacent the acetabulum in preparing the acetabulum for the acetabular cup prosthesis. A handle is releasably connected to the drill guide structure for manipulating the shell during sizing and drilling.