Two patients developed severe hypomagnesemia, hypocalcemia, and hypokalemia as a result of renal wasting of magnesium and potassium shortly after being treated with large doses of gentamicin. When therapy with gentamicin was discontinued renal loss of magnesium and potassium ceased, and serum calcium, magnesium, and potassium returned toward normal. Serum immunoreactive parathyroid hormone levels were inappropriately low during the episodes of hypocalcemia. Both patients represent examples of hypomagnesemic hypocalcemia induced by inappropriate magnesuria, possibly caused by gentamicin. These observations suggest that serum calcium, magnesium, and potassium should be monitored during gentamicin therapy.