A large-scale US survey has shown that the dietary magnesium intake tends to be lower than recommended. The suboptimal intake prevalent among US adults is consistent with the pattern observed in other North American and European surveys. Several factors are discussed, including the waterborne magnesium factor, the loss of magnesium during food refining and the magnesium content of vegetarian diets, as well as various metabolic situations, e.g., hypertension, pregnancy, osteoporosis, drug therapy, alcoholism, stress and cardiac trauma. The benefits of magnesium supplementation among those with sub-RDA intakes are illustrated.