8 patients with hypoparathyroidism have been treated successfully with 1,25-dihydroxycholecalciferol (0-25-1-0 mug/day) for 4-24 mo. Normal serum-calcium levels were restored in all patients, but half the patients required supplementary oral calcium. Treatment reduced serum-phosphorus in all patients, but values remained high in 3, and the renal-tubular reabsorption of phosphate returned to normal in only 2 of the 8 patients. A comparison is made with the effects of oral therapy with massive amounts of calcium salts. The data obtained are discussed in relation to the probable mode of therapeutic action of 1,25-dihydroxycholecalciferol.