Hypomagnesemia can cause hypocalcemia. Because less than 1% of the total body magnesium (Mg) is in extracellular fluids, however, patients may be Mg-deficient despite normal serum Mg concentrations. To determine if hypocalcemia can be seen in patients who have normal serum Mg concentrations but low intracellular Mg, we studied the serum and mononuclear cell Mg contents in 82 alcoholic subjects, 30 of whom had hypocalcemia that could not be explained by other known causes of hypocalcemia. The mononuclear cell Mg content in both hypomagnesemic and normomagnesemic patients with and without hypocalcemia was significantly lower than in normal controls. The serum Mg level did not correlate with the mononuclear cell Mg or serum calcium level, but hypocalcemic patients had a significantly lower mononuclear cell content than normocalcemic patients. Six patients underwent parenteral Mg tolerance testing as an additional measure of Mg deficiency and had increased Mg retention. The serum calcium concentration returned to normal in hypocalcemic patients who were given magnesium intravenously.