The rate of loss of spinal trabecular mineral density (TMD) in postmenopausal women, 49-64 years, was measured during a 52 week walking program. The 8 women who walked were 5.6 +/- 4.4 years past menopause (mean +/- SD) compared to 6.5 +/- 5.1 for 9 nonwalkers. Walkers participated in a progressive walking program for 15-40 min at a heart rate of between 60-85% of maximal age adjusted heart rate, 3 days per week for 52 weeks. Spinal trabecular mineral density was measured using quantitative computed tomography at entry, 6 and 12 months. Pre-exercise heart rate in the walkers decreased 7.8 +/- 1.7 beats per min (mean +/- SEM) (p less than 0.01) from week 0 to week 52, while post-exercise heart rate did not change. Initial spinal mineral density in the walkers was 114 +/- 18 mg/cm3 (mean +/- SD) and 98 +/- 19 mg/cm3 in the controls (NS). Bone loss was 5.6 +/- 1.4% (mean +/- SEM) in the walkers and 4.0 +/- 1.2% in the controls; both of these losses were significantly different from zero (p less than 0.005, p less than 0.01, respectively), but they were not different from each other. Our study shows that a moderate brisk walking program of one year duration does not prevent the loss of spinal bone density in early-postmenopausal women.