The objective of this study was to assess the rate of bone loss and characterize its determinants, among the inhabitants of Taiji, a rural Japanese community. A cohort of 2261 inhabitants aged 40-79 years was established using resident registration in 1992. Fifty men and 50 women in each of four age strata between 40 and 79 years were randomly selected and completed a self-administered risk factor questionnaire. Baseline bone density of lumbar spine and proximal femur was measured by dual-energy X-ray absorptiometry in 1993. BMD was measured again on the same participants in 1996. The rates of change of lumbar spine BMD in men in their 40s, 50s, 60s and 70s were 0.20%, 0.34%, 0.43% and 0.28% respectively. Rates in women were -0.35%, -1.02%, -0.10% and -0.20% respectively. At the femoral neck, rates of change in BMD among men in their 40s, 50s, 60s and 70s were 0.09%, -0.07%, 0.34% and 0.31% respectively. Femoral neck rates of change among women were -0.55%, 0.02%, 0.49% and -0.25% respectively. The rate of change of lumbar spine BMD was -0.24% in premenopausal women with regular periods, -1.99% in premenopausal women with irregular periods and -0.33% in postmenopausal women. Anthropometric measurements at baseline were also related significantly to change in bone density. Baseline weight and height were statistically significant predictors of bone loss rate. These data provide estimates of the rate of bone loss among Japanese men and women aged 40-79 years. They suggest that body build and menstrual function in women are important determinants of bone loss.