Four studies were conducted, each determining the frequency of hypomagnesemia in patients already found to have one abnormal electrolyte determination. Hypomagnesemia occurred in 42% of patients with hypokalemia, 29% of patients with hypophosphatemia, 27% of patients with hyponatremia, and 22% of patients with hypocalcemia. These observations suggest that detection of either hypokalemia, hypophosphatemia, hyponatremia, or hypocalcemia, all of which are routinely available determinations, should alert the clinician to order serum magnesium determinations because of the frequent association of hypomagnesemia with these electrolyte perturbations. Optimally, levels of serum Mg should be determined on a routine basis because of the frequency of the occurrence of hypomagnesemia in hospitalized patients.