Previous studies from these laboratories have demonstrated that uremic biologic fluids contain substances that suppress 1,25(OH)2D metabolism. Among these substances, it was found that uric acid suppresses 1 alpha-hydroxylase activity and synthesis of 1,25(OH)2D in rats. In this study, the effect of uric acid on plasma concentrations of 1,25(OH)2D in patients with renal failure was examined. Nine patients with stable chronic renal failure (serum creatinine, 1.9 to 6.4 mg/dL) were studied. None of the patients received vitamin D supplementation. Plasma concentrations of Ca, P, parathyroid hormone, creatinine, uric acid, 1,25(OH)2D, and 25(OH)D were measured before and 1 wk after the patients received allopurinol, 300 mg daily. Plasma creatinine, Ca, P, parathyroid hormone, and 25(OH)D did not change before or after allopurinol treatment. However, plasma uric acid decreased significantly from 7.3 +/- 0.4 to 4.0 +/- 0.4 mg/dL (P < 0.01) and plasma concentration of 1,25(OH)2D rose from 30.8 +/- 2.7 to 38.2 +/- 4.8 pg/mL (P < 0.01) after the ingestion of allopurinol. Allopurinol itself did not appear to directly enhance 1 alpha-hydroxylase activity in rats. It was concluded that a short-term administration of allopurinol suppresses plasmic uric acid and increases plasma 1,25(OH)2D in patients with chronic mild to moderate renal failure.