Administration of large doses of vitamin D2 brought about a marked increase of 25-hydroxyvitamin D in both the patients with vitamin D dependency and hypophosphatemic vitamin D-resistant rickets. During the administration of vitamin D2, increment of 1,25-dihydroxyvitamin D was marked in hypophosphatemic vitamin D-resistant rickets, but far smaller in vitamin D dependency. In the latter, however, the 1,25-dihydroxyvitamin D reached the level close to the normal adult values. 1 alpha-hydroxyvitamin D3 was found 50 approximately 100 times as effective as vitamin D2 in 2 patients with vitamin D dependency (optimum maintenance dose: 0.05 micrograms/kg/day). It was concluded that 1 alpha-hydroxylation in the renal tubules is markedly defective in the patients with vitamin D dependency, but that a large dose of vitamin D2 is able to cause a definite increase in serum concentration of 1,25-dihydroxyvitamin D resulting in improvement of the rickets.