Aminoglycosides have been reported to cause tetany by inducing renal wasting of magnesium and secondary hypocalcemia. A 9-year-old girl with AIDS was brought to the emergency department because of tetany. She had just finished a 3-week course of intravenous tobramycin for bronchiectasis and had an elevated serum tobramycin trough level 1 week before the onset of tetany. Hypomagnesemia, hypokalemia, and hypocalcemia had developed. The urine revealed inappropriately high levels of magnesium and potassium despite critically low serum levels, with an appropriately low urine calcium level. This pattern is suggestive of renal toxicity due to tobramycin. Over the next several months, she continued to require supplemental calcium, magnesium, potassium, and phosphorus. She also had nephrotic range proteinuria. Aminoglycoside renal mineral wasting has not previously been described in a child.