We made longitudinal measurements of bone mineral density (BMD) in 139 normal women (ages 20-88 yr) at midradius (99% cortical bone) and lumbar spine (approximately 70% trabecular bone) by single- and dual-photon absorptiometry. BMD was measured 2-6 (median, 3) times over an interval of 0.8-3.4 yr (median, 2.1 yr). For midradius, BMD did not change (+0.48%/yr, NS) before menopause but decreased (-1.01%/yr, P less than 0.001) after menopause. For lumbar spine, there was significant bone loss both before (-1.32%/yr, P less than 0.001) and after (-0.97%/yr, P = 0.006) menopause; these rates did not differ significantly from each other. Our data show that before menopause little, if any, bone is lost from the appendicular skeleton but substantial amounts are lost from the axial skeleton. Thus, factors in addition to estrogen deficiency must contribute to pathogenesis of involutional osteoporosis in women because about half of overall vertebral bone loss occurs premenopausally.