A 1-year controlled trial was performed to confirm the effects of 1 alpha-hydroxycholecalciferol (1 alpha-OH-D3) in chronic hemodialysis patients. Initially, a daily dose of 2 micrograms of 1 alpha-OH-D3 was given orally to 24 patients and its placebo to another 24 patients during the first 3 months. For the following 9 months the dose of 1 alpha-OH-D3 or its placebo was reduced to 1 microgram per day in the individual groups. Serum calcium was significantly increased to the normal level after 1 month of treatment and sustained at this level for 1 year. Serum parathyroid hormone was significantly decreased at 3 months. Serum alkaline phosphatase was decreased to the normal level at the 5th month and thereafter. At 2 months serum phosphorus was significantly increased in the 1 alpha-OH-D3 group. None of the patients on 1 alpha-OH-D3 showed increased subperiosteal resorption on X-rays, whereas 8 out of 20 patients on placebo did (p less than 0.002). No adverse effects were seen apart from 3 patients with the 'red eye' of scleral calcification.