The history is presented of a case of postoperative hypoparathyroidism. The first clue which led to the diagnosis was the finding of demineralized bone. Blood analysis showed a low serum Ca and a high serum level, and the diagnosis of hypoparathyroidism, attention was directed to the a-calcification of rats'' teeth and bones following parathyroidectomy, as demonstrated by Erdheim. A theory is presented to explain the mechanism of the a-calcification in hypoparathyroidism. It is pointed out that the localized increase of phosphate ions at the site of calcification resulting from action of the phosphate- phosphorylase enzyme system is very ineffective in the presence of a high serum level of phosphate ions such as occurs in hypoparathyroidism.