Dietary magnesium (Mg) deficiency is more prevalent than generally suspected and can cause cardiovascular lesions leading to disease at all stages of life. The average American diet is deficient in Mg, especially in the young, in alcoholic persons, and in those under stress or with diseases or receiving certain drug therapies, who have increased Mg needs. Otherwise normal, Mg-deficient diets cause arterial and myocardial lesions in all animals studied, and diets that are atherogenic, thrombogenic and cardiovasopathic, as well as Mg-deficient, intensify the cardiovascular lesions, whereas Mg supplementation prevents them. Diuretics and digitalis can intensify an underlying Mg deficiency, leading to cardiac arrhythmias that are refractory unless Mg is added to the regimen. Potassium (K) depletion in diuretic-treated hypertensive patients has been linked to an increased incidence of ventricular ectopy and sudden death. K supplementation alone is not the answer. Mg has been found to be necessary to intracellular K repletion in these patients. Because patients with congestive heart failure and others receiving diuretic therapy are also prone to chloride loss leading to metabolic alkalosis that also interferes with K repletion, the addition of Mg and chloride supplements in addition to the K seems prudent.