Magnesium, the second most abundant intracellular cation, has several critically important roles in the body. In addition to energy production and maintaining electrolyte balance, magnesium is essential for normal neuromuscular function as well as calcium and potassium transport. Evidence suggests that a deficit of magnesium is closely interrelated to potassium deficiency and refractory potassium repletion. Although the consequences of hypokalemia are widely documented and recognized, it is only recently that the importance of magnesium deficiency as a cause of potassium depletion has gained clinical attention. Because of the association between hypokalemia and ventricular ectopy/sudden death, familiarity with the causes of magnesium loss, as well as enhanced identification and treatment, appear to be important.