The use of an imageless computer-assisted navigation system (CANS) in total hip arthroplasty (THA) has the potential to improve the accuracy of implant placement. However, it is difficult to assess how accurately the acetabular cup is placed with the aid of imageless CANS and to evaluate the true cup alignment achieved postoperatively. Eight hips from cadavers and 50 hips from human subjects were studied. The outcomes using imageless CANS were compared to the results of 1) direct bone digitization (DBD) for cadavers, 2) digitization on 3D pelvic-prosthesis models (CT3D) reconstructed from computer tomography scans for cadavers and human subjects, and 3) a conventional mechanical alignment guide (MAG) used for human subjects. The results showed that cup alignments obtained using imageless CANS were reasonably close to those calculated from DBD and CT3D with a mean angle difference smaller than 5°. This suggests that intraoperative cup alignment in THA obtained with imageless CANS is reliable and that CT3D is a valid method for evaluating cup orientation. In comparison to MAG, using the imageless CANS resulted in substantially more accurate cup placement with less variation. In conclusion, the imageless CANS may help to achieve accurate, reproducible intraoperative cup alignment, and CT3D may be useful for evaluating postoperative cup alignment.