Computer Aided Surgery (CAS) systems help surgeons in planning the operation accurately, resulting in costs reducing, clinical outcome improving and enhancing of the health care delivery efficiency. This paper describes the Hip-Op system, which is a novel software for the preoperative planning of total hip replacement. The software uses a fully three-dimensional (3D) internal representation of the patient anatomy, derived from Computed Tomography data. The user interface is based on an innovative visualization paradigm, called Multimodal Display, and represents the anatomical objects by means of multiple views, which are familiar to medical professionals. Besides the original CT slices, also two synthetically generated radio graphs are available, as well as a surface rendering view. The surgeon can load prosthetic models into the planning software environment, selecting them from those available in the implant database. Then he can perform a virtual surgery proceedure in order to plan the proper position and orientation of the prosthesis within the host femur. Analysis modules allow to evaluate the planning from a biomechanical point of view.