Anteversion and vertical tilt of the acetabular prostheses in 50 consecutive total hip arthroplasties were prospectively evaluated during surgery (by the surgeon, using an alignment guide) and radiographically (calculated). From postoperative standardized radiographs vertical tilt was measured directly and anteversion was calculated. The mean error of vertical tilt was 5 degrees (range, 0 degrees - 20 degrees). The mean error of version was 9 degrees (range, 0 degrees - 24 degrees). The reliability of prosthesis placement in a predetermined zone was examined. Although the surgeons believed that all 50 cups were inside this zone, radiographic measurements revealed that 21 of the cups were actually outside. It is concluded that vertical tilt can be reasonably assessed during surgery. Anteversion, however, cannot be accurately assessed during surgery, despite use of the alignment guide.