BACKGROUND In our department, a vacuum bag with two lateral boards used to be utilized in order to maintain patients in the lateral decubitus position during THA. However, we recently changed this modality to a three-point fixation device for the pelvis in order to accurately control the position of the acetabular socket. The purpose of this study was to retrospectively compare the orientation of the acetabular socket when THA was carried out using two different positioning methods. METHODS Group 1 consisted of 66 hips that underwent THA in 1997 using a vacuum bag, which became rigid with suction. Group 2 consisted of 42 hips that were treated in 2000 using a three-point fixation device for the pelvis. This device was designed to able to rigidly fix the bilateral anterior superior iliac spines and the sacrum. Before fixation with the device, the angle between the line aligning the anterior-superior iliac spines and the vertical plane in the lateral decubitus position was measured as the lateral pelvic tilt. The orientation of the fixed acetabular socket was evaluated based on postoperative radiographs. RESULTS The average absolute value from 45 degrees in the abduction angle was significantly lower in group 2 than that in group 1 (p < 0.01). In 18% of the cases in group 1, the anteversion angle was over 30 degrees. In contrast, no such cases were observed in group 2. Postoperative dislocation occurred in six cases (9%) in group 1. In contrast, it only occurred in one case (2%) in group 2. CONCLUSIONS The positioning method using the pelvic fixation device enabled surgeons to more accurately control the position of the sockets during THA comparison to using the vacuum bag. The positioning of the patient is therefore suggested to be an important first step in successfully performing THA procedures.