Norway has a very high incidence of osteoporotic fractures, with substantial regional differences in fracture incidence. The present study evaluated whether there are differences in bone mineral density (BMD) between regions in Norway with differences in fracture incidence. The authors used data collected in four large, population-based, multipurpose studies performed in four regions of Norway during 1994-2001. Distal forearm BMD was measured by single energy x-ray absorptiometry in 10,667 participants aged 40-75 years. Cross-calibration was performed by using the European Forearm Phantom. Mean distal forearm BMD was lower in the urban populations of Tromso, Oslo, and Bergen compared with the rural county of Nord-Trondelag, whereas there was no difference between the rural part of Tromso and Nord-Trondelag. For women, body mass index explained some of these differences. The prevalence of low BMD (z score < or = -1) in Oslo, Bergen, and urban Tromso, compared with Nord-Trondelag, was 1.6-1.7 times higher in men and 1.5-2.0 times higher in women, whereas no significant difference was found between rural Tromso and Nord-Trondelag. In this study, higher BMD was found in rural compared with urban areas of Norway, which might help explain the differences in fracture incidence. There was no apparent north-south gradient in BMD.