Magnesium (Mg) is critical for the function of numerous enzyme systems. Mg deficiency thereby may result in many and varied clinical manifestations. Mg deficiency is common as approximately 10% of patients admitted to city hospitals are hypomagnesemic. Mg deficiency is usually due to losses from the gastrointestinal tract or from the kidney. A serum Mg concentration of < 1.5 mEq/l usually indicates Mg deficiency, however, intracellular Mg deletion may be present despite a normal serum Mg concentration. Acute clinical manifestations of Mg deficiency include neuromuscular hyperexcitability, cardiac arrhythmias, and biochemical abnormalities of hypokalemia and hypocalcemia. Chronic Mg depletion may contribute to hypertension, atherosclerotic vascular disease, altered glucose homeostasis, and metabolic bone disease. Therapy of the acute manifestations usually requires parenteral Mg administration of 24-48 mEq Mg/day for 3-5 days. Long-term Mg repletion may be accomplished by the administration of 300-600 mg of Mg orally/day.