Data to support a relationship between calcium and bone health are a major part of the body of evidence that underlie calcium-related policy in the United States. Examples of these policies include dietary intake recommendations, health claims for calcium and osteoporosis on food labels and an objective to improve calcium intake of the U.S. population in Healthy People 2010. Median calcium intakes among females fall below recommended levels after childhood even when supplemental calcium intakes are included. This is a concern in light of data that support a positive relationship between calcium and bone health. Most of the studies on the calcium-bone relationship have focused on older women, and several have used fracture as the endpoint; a meta-analysis of their results suggests that increased calcium intake is associated with approximately 30% decrease in fracture risk. Studies in children, adolescents and premenopausal women have focused on the relationship between calcium and bone mineral density rather than fracture; most of these also support a positive relationship between calcium intake and skeletal health although some data gaps remain. Calcium appears to be a threshold nutrient (e.g., intakes above a certain level do not result in further benefit to bone). The effect of increased calcium intake on bone density does not appear to persist unless the higher intakes are sustained. There are certain conditions, such as lactation, during which calcium intake does not appear to influence bone. Other factors that may influence the effect of calcium on bone include bone-specific genotypes and physical activity.