A 39-year-old woman presented for evaluation of severe hypomagnesemia of more than 10 years’ duration (average serum magnesium concentration, 1.1; range, 0.5-2.8mg/dL). She has a history of psychiatric disorders, including major depression, attention-deficit/hyperactivity disorder, anorexia nervosa, and past laxative abuse. Hypokalemia has rarely accompanied the hypomagnesemia (average serum potassium concentration, 4.0; range, 3.4-4.8 mEq/L). There is no family history of renal or electrolyte disorders.