The serum magnesium level was measured in 94 consecutive patients admitted to the medical ICU of Los Angeles County/University of Southern California Medical Center over a 2-month period. Sixty-five percent of patients with serum creatinine concentrations of 1.1 mg/dl or less were hypomagnesemic. Of these, one third had hypocalcemia that was corrected with magnesium supplementation. Physicians should be alert to the high incidence of magnesium deficiency in critically ill patients.