A 14-year-old girl with a four-year history of convulsions was found to have congestive heart failure, severe cardiomegaly and hypocalcemia, as well as other signs of idiopathic hypoparathyroidism. Her electrocardiogram showed a prolonged QT interval. Treatment with gluconate calcium and vitamin D cured the heart failure, and normalized the heart size and the QT interval. There was no evidence of any other type of heart disease. Hypocalcemia is one of the few promptly curable causes of cardiomegaly and congestive heart failure in childhood.