Background: Increased dietary calcium intake has been proposed as a population-based public health intervention to prevent osteoporotic fractures. We have examined whether calcium supplementation decreases clinical fracture risk in elderly women and its mechanism of action.Methods: Five-year, double-blind, placebo-controlled study of 1460 women recruited from the population and older than 70 years ( mean age, 75 years) who were randomized to receive calcium carbonate, 600 mg twice per day, or identical placebo. The primary end points included clinical incident osteoporotic fractures, vertebral deformity, and adverse events ascertained in 5 years. Bone structure was also measured using dual x-ray absorptiometry of the hip and whole body, quantitative ultrasonography of the heel, and peripheral quantitative computed tomography of the distal radius.Results: Among our patients, 16.1% sustained 1 or more clinical osteoporotic fractures. In the intention-to-treat analysis, calcium supplementation did not significantly reduce fracture risk ( hazard ratio, 0.87; 95% confidence interval, 0.67-1.12). However, 830 patients (56.8%) who took 80% or more of their tablets ( calcium or placebo) per year had reduced fracture incidence in the calcium compared with the placebo groups (10.2% vs 15.4%; hazard ratio, 0.66; 95% confidence interval, 0.45-0.97). Calcium-treated patients had improved quantitative ultrasonography findings of the heel, femoral neck and whole-body dual x-ray absorptiometry data, and bone strength compared with placebo-treated patients. Of the 92 000 adverse events recorded, constipation was the only event increased by the treatment ( calcium group, 13.4%; placebo group, 9.1%).Conclusion: Supplementation with calcium carbonate tablets supplying 1200mg/d is ineffective as a public health intervention in preventing clinical fractures in the ambulatory elderly population owing to poor long-term compliance, but it is effective in those patients who are compliant.