Bone mineral density (BMD) of the spine and the different regions of interest (ROI) of the hip were measured by dual energy X-ray absorptiometry in 278 healthy Belgian postmenopausal women and 93 postmenopausal type I osteoporotic females in order to: a) determine the normal range for lumbar and hip BMD values; b) define an "hypothetical" fracture threshold in this population; c) determine the preferential region to be considered for clinical use in type I osteoporosis. In the normal subjects, there is a negative relationship (< 0.001) between age or time elapsed since menopause (Tm) and BMD measured at the level of the spine or at the ROI of the hip. For the spine, evidence of a curvilinear relationship was assessed. Regressions of BMD at the hip as a function of age or time elapsed since menopause, were best fitted by a linear relationship. In the population of postmenopausal women who have experienced a vertebral crush fracture, no relationships were observed between spine BMD and age or Tm but the osteoporotic women had a spine BMD significantly lower compared to age-matched normal controls: Z-score = -1.2 +/- 0.6 (mean +/- SD) (p < 0.0001). Fracture threshold calculated as the 90th percentile of spine BMD measured in osteoporotic patients was 0.840 g/cm2, corresponding to the mean value -1 SD for a population of women aged 51 years.