Normal therapeutic dosages of aminoglycosides can cause hypomagnesemia in more than one-third of patients. Hypomagnesemia occurs early in therapy, results in renal Mg++ wasting and may produce hypocalcemia and hypokalemia. Patients who are NPO, eating poorly or not receiving supplemental Mg++ are at high risk for hypomagnesemia. Hypomagnesemia, hypocalcemia and hypokalemia respond to Mg++ replacement therapy. We recommend serial monitoring of serum Mg++ levels in patients receiving aminoglycoside therapy.