Abnormalities of serum Mg may be the most underdiagnosed serum electrolyte abnormality in clinical practice today. The incidence appears to range from 12.5 to 20% on routine determination. Hypomagnesemia is found in 6.9-11% of hospitalized patients on routine determination of serum Mg. A high frequency of hypomagnesemia occurs with other common electrolyte abnormalities: hypokalemia (38-42%), hypophosphatemia (29%), hyponatremia (23%), and hypocalcemia (22%). Routine serum Mg determination would facilitate identification of hypomagnesemia which may be a more frequent contributor to digitalis toxicity than hypokalemia. Identification and treatment of patients at risk for refractory K repletion would be facilitated by routine serum Mg determination. As in our 1976 report to the 2nd International Symposium on Mg Deficiency, it is again our opinion as in 1985 that routine clinical determination of serum Mg represents a continuing unrecognized need. In our opinion routine determination of serum Mg will materially enhance the care of the patient.