BACKGROUND Vitamin D has shown efficacy in the reduction of proteinuria in patients with chronic kidney disease. This study aimed to determine the effect of calcitriol on urinary protein excretion in patients with immunoglobulin A (IgA) nephropathy. METHODS Open-label, non-placebo-controlled, randomized study. METHODS 50 patients with IgA nephropathy were enrolled. The main criterion for inclusion was urinary protein excretion >0.8 g/d after renin-angiotensin system-inhibitor treatment for at least 3 months. METHODS Patients were randomly assigned (1:1) to receive 2 doses (0.5 μg) of calcitriol per week or no treatment for 48 weeks. RESULTS The primary end point was to compare change in 24-hour urinary protein excretion from baseline to last measurement during treatment. METHODS Every 8 weeks, there was measurement of 24-hour urinary protein excretion, serum calcium, serum phosphorus, serum creatinine, and intact parathyroid hormone. RESULTS Measurement of the primary end point showed changes in urinary protein excretion of +21% (from 1.29 to 1.58 g/24 h; 95% CI, -9% to +52%) in the control group and -19% (from 1.60 to 1.30 g/24 h; 95% CI, -42% to +4%) in the calcitriol-treated group. There was a significant decrease in proteinuria in the calcitriol-treated group compared with the control group (difference between groups, 41%; 95% CI, 5%-79%; P = 0.03). The secondary end point of achieving at least a 15% decrease in proteinuria was attained by 7 of 24 (29%) controls and 17 of 26 (65%) of those treated with calcitriol (P = 0.02). No significant differences were observed in decrease in estimated glomerular filtration rate and change in blood pressure between the 2 groups. The incidence of recorded adverse events was similar between the 2 groups. CONCLUSIONS Small and non-placebo-controlled study. CONCLUSIONS The addition of calcitriol to a renin-angiotensin system inhibitor resulted in a safe decrease in proteinuria in patients with IgA nephropathy.