There have been numerous studies of the effects of calcium supplementation, with or without vitamin D, on fractures. Individually, they have not provided clarity regarding calcium's anti-fracture efficacy, though they have established that calcium does have beneficial effects on bone density throughout the skeleton in women. Meta-analysis of these data suggests that total fracture numbers are diminished. However, the data from the 5,500 women involved in trials of calcium monotherapy show consistent adverse trends in numbers of hip fractures (relative risk 1.50, 95% CI 1.06-2.12). Observational data from the Study of Osteoporotic Fractures show a similar increase in risk of hip fracture associated with calcium use. We hypothesize that reduced periosteal expansion in women using calcium supplementation might account for the differences in anti-fracture efficacy of calcium at the hip, in comparison with other sites. Until there are further trial results to clarify this area, the present findings suggest that reliance on high calcium intakes to reduce the risk of hip fracture in older women is not appropriate. In addition, those at risk should be looking to other agents with a proven capacity to prevent hip fractures, such as bisphosphonates.