We hypothesized that use of mechanical acetabular guides for intraoperative alignment leads to variations between the actual and desired implant orientation. Acetabular implant orientation using only the mechanical guide was studied in 78 patients (82 hips) undergoing primary total hip arthroplasty. A computer-assisted navigation system was used to measure alignment and to monitor the orientation of the pelvis during surgery. When using the mechanical guide, there was significant variation in cup alignment from the desired goal of 45 degrees of abduction and 20 degrees of flexion, and this would have resulted in unacceptable acetabular alignment in 78% of hips. With the support system used, there was significant variability in pelvic orientation during surgery. The mean anteversion of the pelvis was an average of 18 degrees from the optimal orientation. These results show a clear need to develop more reliable tools than were used or anatomically based alignment strategies to provide reproducible and accurate acetabular alignment.