There are many reasons to expect magnesium deficiency in patients with chronic congestive heart failure. Medical therapy, neurohormonal activation and decreased dietary intake could all contribute to low concentrations of serum and muscle magnesium. Although the ideal serum level of this electrolyte is not known, multiple studies have documented lower magnesium concentrations in patients with heart failure than in normal persons. Magnesium deficiency could theoretically produce hemodynamic deterioration and ventricular arrhythmias. These complications have been observed in animals and in patients without heart failure, and magnesium repletion has reversed the adverse effects of hypomagnesemia in some patients. However, the consequences of chronic depletion of the electrolyte have not been adequately evaluated. Because of the high incidence of sudden death in patients with severe congestive heart failure, well designed investigations to determine the importance of magnesium are needed.