Objective - To assess the prevalence of primary hip osteoarthritis (OA) in Iceland. To compare the prevalence of primary hip OA in Iceland with published rates of primary hip OA for related Scandinavian populations. Methods - Roentgenographs were examined of 1530 Icelandic people 35 years or older (653 males, 877 females) subjected to colon radiography during the years 19901996. The radiographs examined represent approximately 40% of all colon radiographs taken in Iceland during this period. After exclusion of non-primary hip OA cases, the minimum hip joint space was measured with a mm ruler. Presence of hip OA was defined as a minimum joint space of 2.5 mm or less on an anteroposterior radiograph. Intraclass correlation coefficients for inter and intraobserver variability of assessment of mm joint space were 0.91 and 0.95, respectively. Results - Of the 1517 people included, 227 hips in 165 patients (77 men, 88 women) were diagnosed as having radiological primary hip OA. The mean age at colon examination for these patients was 68 (35-89) years. The overall prevalence of coxarthrosis among all examined patients 35 years and older was 10.8% (12% for men, 10% for women), rising from 2% at 35-39 years to 35.4% for those 85 years or older. If the population structure (age and sex distribution) for those older than 35 years in Iceland was used to standardise prevalence for both Iceland and south Sweden (using previously published data for south Sweden), the age and sex standardised prevalence of hip OA for those older than 35 years in Iceland was 8%, compared with 1.2% for south Sweden. Conclusions - The prevalence of radiological primary hip OA is very high in Iceland, and in excess of fivefold higher than the prevalence found by using similar techniques in studies on related populations in southern Scandinavia. The rate difference is particularly notable for those younger than 70 years.