One hundred twenty patients undergoing total hip arthroplasty (THA) for unilateral hip osteoarthritis or femoral neck fractures were randomized to receive acetabular cup positioning using either a mechanical alignment guide (MAG) (n=40) or a novel method based on preoperative three-dimensional CT (3D-CT) planning relative to the native acetabular rim (n=80). The primary outcome was the percentage of cups positioned within 5° of the target radiographic inclination (40°) and anteversion (20°). Secondary outcomes included alignment within the Grammatopoulos aiming zone (40°/20° ± 10°), radiographic measures of inclination and anteversion, and variability in placement. Outcomes were assessed postoperatively with radiographs and CT scans, and patients were followed for a minimum of two years. Overall, the results of the study revealed that 86.2% of cups in the 3D-CT group met the primary accuracy target compared to only 32.5% in the MAG group, with the 3D-CT group also achieving 100% alignment within the Grammatopoulos zone. These findings suggest that 3D-CT-guided positioning significantly improves the precision of acetabular cup placement in THA.