This retrospective study was designed to investigate whether sub-clinical acetabular dysplasia, defined by a reduced centre-edge angle of Wiberg, was associated with a younger age at hip arthroplasty in patients with idiopathic osteoarthritis (OA). Fifty-four patients with 69 performed arthroplasties and no previous referral for hip dysplasia were selected from a list of consecutive recipients of hip endoprostheses due to idiopathic OA. The centre-edge angle was measured from standard pelvic radiographs taken a mean of 5.1 years prior to the endoprosthesis operation when there were minimal signs of hip OA. The age at which hip arthroplasty was carried out was compared between those patients with low (20 degrees - 35 degrees ) and those with high (>or= 35 degrees ) centre-edge angles. The mean age at hip arthroplasty was significantly younger in the group with centre-edge angles of 20 degrees - 35 degrees (65.6 years) compared with those with centre-edge angles >or= 35 degrees (69.2 years). These results suggest that sub-clinical acetabular dysplasia was associated with a younger age at hip arthroplasty in idiopathic OA.