The effects of a small dose of calcitriol (less than or equal to 0.50 micrograms/day) on parathyroid and renal function, bone histomorphometry, and aluminum (Al) metabolism were studied in a randomized double blind study of 30 patients with predialysis chronic renal failure. The patients were followed at least monthly for 8 months. Serum Al levels were measured, and transiliac bone biopsies, double labeled with tetracycline, were obtained at both the beginning and end of the 8-month treatment period. Serum calcium and ionized calcium concentrations increased in the treatment group, and the calcitriol dosage had to be reduced in 8 patients at least once because of hypercalcemia. Calcitriol treatment did not significantly influence either serum A1 levels or the presence of stainable Al in bone. Serum PTH, urinary cAMP excretion, and bone resorption indices decreased in the treatment group, indicating suppression of parathyroid hyperfunction. Throughout the study renal function decreased at a similar rate in both groups, suggesting that calcitriol treatment had no depressive effect on renal function. We conclude that a low dose of calcitriol may be used to preserve or even restore bone metabolism in patients with predialysis chronic renal failure if serum calcium is closely followed and hypercalcemia promptly treated.