The aim of this study was to elucidate perimenopausal bone loss in relation to menstrual conditions and to investigate the long-term effect of menopause on bone loss in aged women. The rate of change in bone mineral density (BMD) was measured twice at an exact interval of 12 months by dual-energy X-ray absorptiometry (DXA) at the lumbar spine in 176 pre- and postmenopausal healthy women 41-65 years of age. Serum follicle-stimulating hormone, intact and N-fragment osteocalcin (OC), three types of vitamin D3, parathyroid hormone (PTH), and calcitonin were also determined. Women who exercised regularly or had anatomical changes at the lumbar spine were excluded from this study. The subjects were divided into eight groups based on their menstrual status and years since menopause. Annual bone loss at the lumbar spine of premenopausal women with regular menstruation was -0.2+/-1.9% (95% confidence interval, -0.9 approximately -0.4%) and was not statistically different from zero, while that of women with irregular menstruation or at menopausal transition was -2.1+/-3.4% (-3.4 approximately -0.8%), and -3.3+/-2.3% (-5.2 approximately -0.3%), respectively, and was significantly different from zero. Serum OC levels of women at menopausal transition were significantly higher than those of women with regular menstruation, suggesting that bone loss had commenced in these women. The rate of annual change in BMD of women who were menopausal for 1-3, 4-6, 10-12, and more than 13 years was -3.1+/-4.0% (-4.7 approximately -1.5%), -1.2+/-2.6% (-2.2 approximately -0.2%), -1.0+/-3.0% (-2.3 approximately -0.3%), and -2.3+/-2.1% (-3.7 approximately -1.0%), respectively, and was significantly less than zero. But the annual bone loss of women who were menopausal for 7-9 years was -1.5+/-2.6% (-3.0 approximately -0.1%) and was not statistically significant from zero. These results indicate that postmenopausal women lose BMD in two phases. The early bone loss is rapid and commences during irregular menstruation, then is attenuated within 6 years after the onset of menopause. The second bone loss commences after the attenuation of the first bone loss. Among bone metabolic hormones, intact PTH alone showed an age-related increase and was suggested as being a causal factor of bone loss in women who were menopausal for 13 years or more.