We examined the effect of exercise training and detraining on bone mineral density (BMD) in postmenopausal women with osteoporosis. Thirty-five postmenopausal women with osteoporosis, aged 53-77 years, were randomly assigned to three groups: a control group (n = 20), a 2-year exercise training group (n = 8), and an 1-year exercise training plus 1-year detraining group (n = 7). Exercise training consisted of daily brisk walking and gymnastic training. Calcium lactate, 2.0 g, and 1alpha-hydroxyvitamin D3, 1 microg were supplied daily to all subjects. No significant differences in initial lumbar BMD, measured by dual-energy X-ray absorptiometry (DXA) were found among the three groups. The mean percent change in BMD compared with the baseline was significantly higher at 1 and 2 years in the exercise training group and at 1 year in the detraining group than in the control group, and did not differ significantly at 2 years between the detraining and control groups. These findings indicate that our exercise training program led to a significant increase in lumbar BMD in postmenopausal women with osteoporosis compared with the control, but that the BMD reverted toward a level that was not significantly different from the control with detraining. Continued exercise training is needed to maintain the bone mass gained through exercise training.