Aims: To establish whether the Ôidealñ concentric positioning of acetabular components within the acetabular cement mantle is achieved during routine cemented total hip arthroplasty. In vitro studies recommend concentric placement of the acetabular component. There are however no in vitro studies on acetabular component positioning. Methods: Radiographs of 100 primary cemented total hip replacements were studied. Acetabular component positioning and cement mantles were assessed with respect to implant type, grade of surgeon and operated side. Results: 78% of the components were eccentrically placed, with increasing cement mantle thickness from zones 1 to 3. Concentricity occurred in only 13% of the Charnley Ogee, and 28% of the IP Lubinus components. The Charnley Ogee was more superiorly eccentric than the IP Lubinus (p< 0.001). The IP Lubinus was generally more open than the Charnley Ogee and Stanmore (p=0.053). Surgical grade affected neither cement mantle nor opening angle. Conclusions: This study indicates that the ideal component position in the acetabulum is very difþcult to achieve consistently in most patients, using the two commonly used cemented prostheses in this study. The practical difþculty of concentric component positioning while obtaining simultaneous pressurisation is illustrated