To determine whether increased Ca intakes can prevent rickets in a susceptible group of children living in a rickets-endemic area of Bangladesh, we conducted a 13-month long, double-blind, clinical trial with 1-to 5-year-old children who did not present with rickets but ranked in the upper decile of plasma alkaline phosphatase (AP) activity of a screening cohort of 1,749 children. A total of 158 children were randomized to a milk-powder-based dietary supplement given daily, 6 days/week, and providing either 50, 250, or 500 mg Ca, or 500 mg Ca plus multivitamins, iron, and zinc. Upon initial screening, 194 healthy children presented with no rachitic leg signs and had serum AP in the upper decile (>260 u/dl) of the cohort. When 183 of those subjects were re-screened after a 7-month pre-trial period, 23 (12.6%) had developed rachitic leg signs, suggesting an annual risk of 21.5% in this cohort. Of those still not presenting with leg signs and completing 13 months of dietary intervention, none showed rachitic leg signs, none showed significant radiological evidence of active rickets, and all showed carpal ossification normal for age after that intervention. These results are consistent with even the lowest amount of supplemental Ca (50 mg/day) being useful in supporting normal bone development in this high-risk population.