Twelve patients with chronic renal failure (CRF) requiring dialysis, 7 of whom were hypercalcaemic, were treated with 24,25-dihydroxycholecalciferol [24,25(OH)2D3; 10 μg daily for 3 months] or placebo and the treatment then crossed over for a further 3 months. Treatment with 24,25(OH)2D3 was associated with small but significant increments in the fractional rates of absorption of calcium (p < 0.01) and phosphate (p < 0.05) measured by a combined radioisotope test, but even during treatment, the absorption of both calcium and phosphate remained subnormal. A significant fall in immunoreactive parathyroid hormone (i-PTH) levels occurred during treatment (p < 0.05) in the absence of significant changes in plasma ionised calcium and plasma phosphate levels. In 7 patients, treatment with 24,25(OH)2D3 50% suppression of i-PTH levels) at similar levels of ionised calcium. 24,25(OH)2D3 may have a role in the treatment of renal osteodystrophy, especially in patients with hypercalcaemic hyperparathyroidism