Vitamin D metabolites 25-OH-D3 (calcifediol) and 1,25-(OH)2-D3 (calcitriol) were measured in plasma in 16 patients with advanced chronic renal failure during treatment with a protein-restricted diet for 6 months. The glomerular filtration rate (GFR) decreased only marginally, from 8.3 to 7.9 ml/min, during the study while there was a significant decrease of serum urea levels after the initiation of the protein-reduced diet. Calcitriol levels rose significantly (p < 0.05) after 3 months from 17.1 to 27.7 pg/ml but fell after another 3 months to nearly their initial values, 15.3 pg/ml. The serum phosphate levels changed during the same periods from 1.99 to 1.67 to 1.93 mmol/l. There were significant inverse correlations between the calcitriol and phosphate levels at the start and after 3 months but not after 6 months. There was also a significant correlation between the changes in calcitriol and initial GFR. A subgroup of patients with decreased or unchanged calcitriol concentrations during dietary treatment had significantly higher serum phosphate and creatinine and significantly lower standard bicarbonate concentrations. Multivariate regression analysis for the pooled set of data with calcitriol as the dependent variable showed significant values for GFR (p < 0.02), body mass index (p < 0.02), and serum phosphate concentrations (p < 0.04). These results show the importance of phosphate control and renal function for the regulation of calcitriol synthesis.