Bone mass was measured prospectively in 73 women during the period immediately after menopause. By comparing the rates of loss at three skeletal sites, we assessed the protective effects of calcium supplements given alone or combined with low-dosage estrogen therapy. After 2 years of follow-up, spinal trabecular mineral content, measured by quantitative computed tomography, decreased by a mean of 9.0% (p = 0.002 compared with baseline) in untreated women and a mean of 10.5% (p = 0.0001) in women given calcium supplements alone. By contrast, women given conjugated estrogens, 0.3 mg/d, with calcium supplements showed an insignificant increase of 2.3%. Significant losses of a lesser magnitude were seen in the appendicular cortical skeleton of women not receiving therapy and in those receiving calcium alone, but no significant changes were observed in women receiving estrogen with calcium.