Some authors have reported that vitamin D treatment worsens renal function. We studied 10 patients with stable renal function, who were given calcitriol (0.5 micrograms/day) for a 4-month period. Creatinine and inulin clearance were performed at the beginning and at the end of the treatment. Although serum creatinine increased and creatinine clearance decreased, inulin clearance did not show significant variations. Furthermore, serum creatinine fell to the baseline value within 60 days after discontinuation of vitamin D therapy. The increased serum creatinine may be explained by an augmented release from muscular tissue, probably due to the improvement of uremic myopathy induced by calcitriol.