A controlled study of the effects of the potent vitamin-D metabolite, 1, 25-dihydroxycholecalciferol (1,25[OH]2D3), and vitamin D3 was done in 18 non-dialysed patients with chronic renal failure (C.R.F.). Patients with a creatinine clearance below 35 ml/min and mild renal osteodystrophy were selected. After 6 months' observation of the spontaneous course the patients were randomly allocated to 6 months' oral treatment with either 1, 25 (OH)2D3 or vitamin D3 in initial daily doses of 1microgram and 4000 I.U., respectively, combined with 0.5 g calcium. 1,25(OH)2D3 quickly corrected hypocalcaemia, reduced serum-alkaline-phosphatases and serum-immunoreactive-parathyroid-hormone, and more than doubled the urinary excretion rate of calcium. D3 had similar, but less pronounced effects. 7 out of 8 patients on 1,25(OH)2D3, developed hypercalcaemia which necessitated a reduction in dosage. None of the patients on D3 treatment developed hypercalcaemia. The percentage fall in creatinine clearance was greater during treatment than before treatment in all patients on 1, 25 (OH)2D3 (P less than 0.01) and in 7 of 9 patients on vitamin D3 treatment (though the group change here was not significant). Deterioration of renal function is a major limitation of the clinical use of 1, 25(OH)2D3 and D3 in non-dialysed patients with C.R.F. In fact, the decrased formation of 1, 25(OH)2D3 seen in C.R.F. might protect renal function at the expense of abnormalities in mineral metabolism.