The authors studied the clinical characteristics of primary and post-operative hypoparathyroidism in 39 patients. Laboratory follow-up data were compared under two different treatment programs using either AT 10 or 25 Hydroxycholecalciferol (25 OHCC). Clinical analysis revealed the atypical characteristics of primary hypoparathyroidism. From a therapeutic standpoint, AT 10 and 25 OHCC were equally effective in provoking a return to normal plasma calcium levels, except in complex cases of vitamin D resistance. 25 OHCC proved much easier to manipulate than at 10 and offered a higher security with respect tothe risk of hypercalcemia. The biological activity of 25 OHCC seems to differ from that of AT 10, especially regarding phosphorus metabolism.