Three adolescents with uraemic osteodystrophy were treated for 7 months with daily oral doses of 1α-hydroxycholecalciferol (0.25–10.0 µg). All the patients were hypocalcaemic and had high serum levels of alkaline phosphatase before the treatment. A rapid rise in serum calcium and a slow, but pronounced decline in serum alkaline phosphatase concentration were observed during the period of treatment. lα-Hydroxycholecalciferol induced hypercalcaemia in two situations. In both cases the hypercalcaemia was transient, serum calcium being normalized in a few days by withdrawal of the drug. Withdrawal of the drug also in other situations resulted in a fall in serum calcium concentration in a couple of days. This suggests that 1α-hydroxycholecalciferol should be given daily or every other day. The response did not subside during 7 months of treatment. On the contrary, the maintenance dose necessary to keep serum calcium constant was smaller than the initial dose necessary to normalize serum calcium. Bone mineral content, estimated by photon absorptiometry, rose. The rachitic bone lesions seen radiologically were significantly ameliorated during the treatment.