Two hundred and twenty-four women (74 pre-, 90 peri-, 60 post-menopausal), aged 46-59 years, from a population-based cohort participated in a longitudinal study of bone mineral density (BMD). BMD was measured by dual-energy X-ray absorptiometry (DXA) at the lumbar spine and femoral neck and the time between bone scans was on average 25 (range 14-41) months. The aim of the study was to assess changes in BMD in relation to changes in normal menopausal status. During the study period women who were between 3 and 12 months past their last menstrual period (n = 22, late perimenopausal) at the time of the second bone scan had a mean (SE) annual change in BMD of -0.9% (0.4%) at the lumbar spine and -0.7% (0.6%) at the femoral neck (both p < 0.05 compared with women who remained premenopausal). In the women who became postmenopausal (n = 42) the mean annual changes in BMD were -2.5% (0.2%) at the lumbar spine and -1.7% (0.2%) at the femoral neck (both p < 0.0005), and in the women who remained post-menopausal (n = 60) they were -0.7% (0.2%) per year and -0.5% (0.3%) per year respectively (both p < 0.05), compared with women who remained premenopausal. In the 1-3 years after the final menstrual period (FMP) there was greater bone loss from the lumbar spine than the femoral neck (p < 0.05). In women who were menstruating at the time of the second bone scan and whose FMP could be dated prospectively (n = 35), higher baseline oestradiol levels were associated with less lumbar spine bone loss (p < 0.005). In the women who remained postmenopausal there was an association between baseline body mass index (BMI) and percentage change per year in femoral neck BMD (p < 0.05), such that women with higher BMI had less bone loss. In conclusion, during the time of transition from peri- to post-menopause, women had accelerated BMD loss at both the hip and spine.