The influence of 5,6-trans-25-hydroxycholecalciferol on renal osteopathy was investigated in a total of 132 patients in 26 dialysis centres. Various doses were used, the average being 4000-6000 IU/day. In 32 patients a daily dose of 6000-9000 IU was used. The average individual duration of treatment was 276 days with a maximum of 910 days. Histologically there was an improvement in the renal osteopathy in 55.9% of evaluable cases (n = 34) and in 25.3% there was no deterioration. Radiographically these results were found in 21% and in 70.5% of evaluable cases (n = 105). Serum calcium increased in 46.6% of cases (n = 131), remained the same in 32.8% and decreased in 20.6%. The changes in alkaline phosphatase were similar : it dropped in 42.1% of patients, remained the same in 28.1% and rose in 29.8%. Immunoreactive parathormone which was invariably raised at the beginning of treatment (n = 36), fell in 25.0%, remained the same in 44.4% and rose further in 30.6%. The clinical symptoms of renal osteopathy which had been present in 57 patients improved in 51.0%, remained the same in 46.0% and deteriorated in 3.0%. Signs of intolerance and side effects were rare. Severe hypercalcaemia did not occur.