In an uncontrolled open study 13 patients with moderate to preterminal renal failure were treated with low doses (average 0.36 micrograms/day) of calcitriol up to the time of renal transplantation, which was performed before dialysis had been initiated. A transiliac bone biopsy was obtained both at the start and at the end of the treatment period, the latter coinciding with renal transplantation. All patients who started calcitriol treatment at a creatinine clearance (Ccr) above 30 ml/min had normal bone histology at the time of transplantation, but this was not observed when calcitriol treatment was started at Ccr below 30 ml/min. The study suggests that full benefit of calcitriol at the bone level is obtained only if prophylactic administration is started early in the course of renal failure.