OBJECTIVE:To assess the effects of variations in hip morphology on prevalence, clinical severity and progression of hip osteoarthritis.METHODS:From 2007 to 2009, we conducted a study of 242 patients aged between 40 and 75 years with symptomatic lower limb osteoarthritis, as part of a population-based osteoarthritis cohort study in France. Standard radiographs of both hips were obtained at baseline and at three years. The progression of hip osteoarthritis was evaluated according to the radiological Kellgren-Lawrence score (KL) and clinical severity from the scores on a Visual Analogic Scale (VAS) for pain recorded yearly. Five measures were used to describe hip morphology: centre edge angle, acetabular index (AI), vertical centre anterior angle, acetabular depth and neck-shaft angle.RESULTS:Of the 484 hips studied, 205 (42%) showed osteoarthritis at baseline and 16 (11 right and 5 left) underwent joint replacement during the follow-up. AI was the morphological measure most consistently and strongly associated with radiographic osteoarthritis at baseline (odds-ratio=1.05, 95% CI: 1.01-1.08 per degree of angle change), clinical severity (correlation coefficient with VAS during 3 years=0.15, P=0.004), radiological progression (odds-ratio=1.05, 95% CI: 1.00-1.10 per degree) and joint replacement (hazard ratio=1.18, 95% CI: 1.07-1.29 per degree).CONCLUSIONS:Acetabular obliquity and especially AI is strongly, and likely causally, associated with the existence, severity and progression of hip osteoarthritis.