Serum magnesium levels were determined by atomic absorption spectrometry on 5,100 consecutive patients seen at a diagnostic clinic. The average value was 1.78 ± 0.15 mEq/liter with 90% of the values between 1.53 and 1.99 mEq/liter. The prevalence of various clinical conditions found in those patients with abnormal serum magnesium levels was compared with their prevalence in 500 randomly selected patients with magnesium levels between 1.53 and 1.99 mEq/liter. Hypo- and hypermagnesemia were more frequently encountered in seriously ill patients requiring hospitalization than in ambulatory patients. Diabetes mellitus was the most common condition associated with hypomagnesemia. Hypomagnesemia was also observed in increased frequency in patients on diuretic therapy. Hypermagnesemia with values above 2.08 mEq/liter was encountered in uremic patients and in patients receiving magnesium therapy, usually in the form of antacids.