During a two-year study, we examined the effect of calcium supplementation on postmenopausal bone loss in 43 women in the early postmenopausal period who were assigned to one of three treatment groups: percutaneous 17 beta-estradiol (combined with progesterone during the second year), oral calcium (2000 mg daily), and placebo. All participants were examined every three months. Bone mineral content in the forearm (measured by single-photon absorptiometry) and in the entire body and spine (measured by dual-photon absorptiometry) remained constant in the estrogen-treated group but decreased significantly in the groups receiving calcium and placebo. In the calcium-treated group, we observed a tendency toward a slowed loss of compact bone (in the proximal forearm and total skeleton) as compared with the placebo group, while the rate of loss of trabecular bone (the distal forearm and spine) was the same as in the placebo group. Our preliminary data suggest that calcium supplementation in the dosage we used is not as effective as estrogen therapy for the prevention of early postmenopausal bone loss. Calcium supplementation may have had a minor effect on the loss of cortical bone, but it had no effect on the trabecular bone.