Magnesium (Mg) deficiency is a common finding in critically ill patients. Mg deficiency results primarily from gastrointestinal or urinary Mg losses, but malnutrition and decreased dietary Mg intake may hasten the development of Mg depletion. In our medical intensive-care unit, we have found hypomagnesemia in 65% of patients with normal serum creatinine concentrations. The prevalence of normomagnesemic Mg deficiency in critically ill patients may be even higher and may contribute to the pathogenesis of hypocalcemia, cardiac arrhythmias and other symptoms of Mg deficiency.