We measured the bone mineral density (BMD) at various skeletal sites (total body, hip, anteroposterior [AP] and lateral [lat] spine, and forearm) in a large population-based cohort of women aged 31-89 years (the OFELY cohort), and results were analyzed according to age and postmenopausal years. A significant apparent bone loss was found before the menopause in cancellous bone, i.e., at the lat spine and Ward's triangle (-10%; p < 0.05-0.001). Cross-sectional analysis indicated that, after the menopause, apparent bone loss was accelerated within the 10 years following menopause, continued thereafter at all sites except the AP spine, and was again accelerated in elderly menopausal for more than 25 years. Between 30 and 80 years, BMD decreased by 15 to 44% (T score -1.6 to -3.4) according to the site. The amount of apparent bone loss was highest at the Ward's triangle when expressed in percentage (44%) and at the mid- and distal radius when expressed in number of standard deviations from the peak bone mass (-3.4). As a result, the percentage of women classified as osteoporotic according to the World Heath Organization, i.e., with a T score < or = -2.5, varied substantially from site to site and was highest at the radius (37% and 46%) and lateral spine (25-31%), intermediate at the Ward's triangle, AP spine, and whole body BMD, and lowest at the whole body bone mineral content, femoral neck, and trochanter (10-12%). In conclusion, this cross-sectional but large study suggests that there is a moderate apparent premenopausal bone loss that occurs only at cancellous bone sites and that apparent bone loss is accelerated at most skeletal sites after the age of 75 years. Because of the highly variable coefficient of variation of the peak bone mass at various skeletal sites, the percentage of postmenopausal women identified as being osteoporotic varies widely according to the site of measurement.