To assist surgeons performing pelvic osteotomies for the treatment of dysplastic hips, an image guided freehand navigation system has been developed. Preoperative computed tomographic scan images are presented in various ways to the surgeon together with real time display of the instruments and surgical action on the computer screen. The system supports the preoperative plan and provides optimized control of surgical action. The main focus of the image guidance has been placed on the execution of the different required cuts and the reorientation of the acetabular fragment. Special attention also has been given to the development of a sophisticated surgeon-machine interface. Fourteen surgeries have been performed with image guidance so far. The visualization aids provided by the system are able to help reduce potential risk and thus increase safety and accuracy for this difficult class of surgical interventions.