Osteoporotic fractures have many sources. Low bone mass is one such, and inadequate calcium intake, in turn, is one of the causes of low bone mass. Calcium intake may be inadequate because it is low in its own right or, even if 'normal', it may not be sufficient to compensate for exaggerated obligatory losses. Inadequate calcium intake may cause bone mass to be low either because calcium intake during growth limits achievement of genetically programmed skeletal mass, or because low intake later in life aggravates involutional loss, or both. Ensuring a generous calcium intake throughout life will prevent both of these consequences. However, it is important to stress that even a calcium surfeit will not prevent or reverse bone loss due to inactivity, gonadal hormone deficiency, alcohol abuse or, indeed, any other factor. Calcium is a nutrient, not a drug. The only disorder it can be expected to alleviate is calcium deficiency. However, the evidence suggests that calcium deficiency is prevalent among Western populations, particularly in North America, and that it thereby contributes substantially to their osteoporotic fracture burden. This component of that burden is therefore entirely preventable.