To determine the long-term effect of calcium supplementation on bone density, 84 elderly women (54-74 years) more than 10 years past the menopause were studied for 4 years as part of a follow-up study of a randomized, double-masked, placebo-controlled trial. The placebo group who did not take calcium supplements at all during the 4-year study (control group, n = 21) served as a comparison with the treated group who took calcium supplements for 4 years (calcium supplement group, n = 14). We also studied subjects who were treated for 2 years with calcium supplements and then ceased taking them (non-compliant group, n = 49). The changes in bone density at the lumbar spine, hip and ankle sites, current calcium intake and activity were monitored. Over the 4 years the calcium supplement group (mean calcium intake 1988 +/- 90 mg/ day) did no lose bone at the hip and ankle site. The control group (mean calcium intake 952 +/- 109 mg/day) lost significantly more bone than the calcium supplement group at all sites of the hip and ankle. No overall bone loss was seen at the spine, in either group, over the 4 years of this study. Between years 2 and 4 the noncompliant group (mean calcium intake 981 +/- 75 mg/day) lost significantly more bone at all sites of the ankle than the calcium supplement group. Therefore, calcium supplementation produces a sustained reduction in the rate of loss of bone density at the ankle and hip sites in elderly postmenopausal women. Increasing dietary calcium intake in women should be the aim of a public health campaign.