Oxalic acid is one of the well-known uremic toxins that participates in the pathogenesis of uremic syndrome. Secondary hyperoxalemia is a common feature in patients with chronic renal failure, but oxalate removal is not adequately accomplished by renal replacement therapy. In our series of patients, the plasma level of oxalic acid was significantly elevated, while the plasma vitamin C was in the normal range or in the upper margin of the normal range. The peritoneal clearance of oxalic acid was significantly lower in comparison to the peritoneal clearance of urea. Peritoneal clearance and peritoneal transfer of oxalic acid and other examined parameters increased using dialysis solution containing 2.5% glucose in comparison to dialysis solution containing 1.5% glucose. The significant hyperoxalemia of our patients persisted despite the relatively high peritoneal transfer of oxalic acid during continuous ambulatory peritoneal dialysis. The clearance of oxalic acid related to the clearance of urea was 58.1% during hemodialysis, 74.2% during postdilution hemofiltration, and 69.0% during postdilution hemodiafiltration. The sieving coefficient of oxalic acid during postdilution hemofiltration was 74.0% of urea sieving coefficient. The most significant decrease of plasma oxalic acid was observed during postdilution hemodiafiltation (63.3%). These results suggest that currently, renal replacement therapy is not effective enough for a permanent reduction of plasma oxalic acid.