Acetabular cup orientation is a key factor determining hip stability, and standard mechanical guides have shown little help in improving alignment. An in vitro study was carried out to compare the accuracy and precision of a new gravity-assisted guidance system with a standard mechanical guide. Three hundred ten cups were impacted by 5 surgeons, and the final cup orientation was measured. With the new guide, the average error in anteversion was 0.4 degrees , compared with 10.4 degrees with the standard guide and 0.3 degrees and -4.7 degrees , respectively, for abduction angles. The average time required for orienting the cups was similar for both guides. The accuracy and reproducibility obtained with the new guide were better (P < .0001). These good results would require a clinical validation.