We report a case of magnesium deficiency associated with acetaminophen abuse. Metabolic studies showed that the patient's fractional excretion of magnesium was 12% when her serum magnesium level was 1.0 mg/dL. Renal biopsy demonstrated severe tubulointerstitial disease. The significant wasting of magnesium at a time when serum magnesium levels were depressed suggests renal magnesium wasting syndrome. This defect was temporally related to abuse of acetaminophen.