Hypocalcemia is a relatively uncommon but reversible cause of congestive heart failure. A 39-year-old woman with hypocalcemia due to untreated hypoparathyroidism presented with congestive heart failure and a left ventricular ejection fraction of 25 percent. She had no evidence of underlying cardiac disease. The serum calcium level was normalized within one week, associated with complete clearing of the signs and symptoms of heart failure and a twofold increase to 50 percent in the ejection fraction. This is the first patient without underlying myocardial disease in whom significant improvement of left ventricular function has been quantitatively assessed prior to and following correction of hypocalcemia.