The purpose of this study was to: (I) assess the prevalence of hip osteoarthritis (OA) in Iceland and compare it with that in Southern Scandinavia, (II) determine the incidence of total hip replacement (THR) for primary OA in Iceland, (III) compare two different methods for defining radiographic hip OA, (IV) assess in a population-wide study in Iceland the genetic contribution t hip OA leading to THR, and (V) perform a genome-wide scan of a large Icelandic family to identify a chromosomal susceptibility locus for hip OA leading to THR. many Icelandic patients with hip OA have been well aware that this disease "goes in the family". by examining a large proportion of all Icelandic colon radiographs taken 1990-1996 the prevalence of radiographic hip OA in Iceland was found to be at least five-fold higher compared to Swedish and Danish studies that have used the same methods. A comparison of two methods for estimating hip OA from colon radiographs showed that a simple quantitative method of measuring joint space was more reliable than a qualitative method. The age-standardized incidence of THR for primary hip OA in Iceland between 1982 and 1996 was estimated and found to be about 50 percent higher than for Sweden. The higher Icelandic prevalence of hip OA may explain most of this difference. To investigate the contribution of heritability to hip OA leading to THR, information from two population-wide database in Iceland was combined: A national registry of THR between 1972 and 1996, and a genealogy database of all Icelandic genealogy records for the last 11 centuries made available by deCode Genetics. The genetic contribution to THR for OA was assessed by (a) identifying familial clusters of THR for OA, (b) applying the minimum founder test (MFT) to estimate the minimum number of ancestors to account for all patients with THR for OA, compared to the average number of founders for control lists, (c) calculating an average pairwise kinship coefficient (KC) for the patient and control lists, (d) estimating the relative risk (RR) for relatives of patients with THR for OA. A large number of familial clusters of patients with THR for OA were identified. MFT showed that OA patients descended from fewer founders than the control groups. The average pairwise KC among patients with OA was greater than in the population. RR for siblings of THR for OA patients was 3.05 (2.52, 3.10). Icelandic patients with THR for OA are thus significantly more related to each other than are matched controls. These findings support a significant genetic contribution to a common form of OA and encourages the search for genes conferring an increased susceptibility to OA. New techniques now make it possible to search the whole human genome for chromosomal susceptibility loci associating with OA. A genome wide scan was done to identify susceptibility loci for hip OA leading to THR, using DNA from a large Icelandic family with a very high prevalence of primary hip OA. A genome locus with a lod score of 2.58 was identified on chromosome 16p. A similar locus has been reported on from England. This is the first instance where what may be the same susceptibility locus for OA is independently described in two different populations with hip OA. We have identified other families with hip OA which link to the studied family and are continuing an expanded genome-wide scan. Continued studies of the kind outlined here will clarify the complex genetic background of OA and identify genetic variation associated with the disease. In addition to improving our understanding of the pathogenesis of OA and identifying new molecular targets for treatment, this will allow a better insight into the interactions between genetic background and environmental factors that initiate and drive OA.