A new hemodiafiltration technique, acetate free biofiltration (AFB), has been recently introduced in the treatment of chronic uremia. It is performed with a buffer free dialysate and a simultaneous infusion in post-dilution mode of a sodium bicarbonate solution (concentration 145-166 mEq/l; infusion rate 1.2-2 l/h). A polyacrylonitrile hollow fiber AN69 HF 1.2 sqm dialyzer is employed. In acute studies pCO2 remains stable throughout the treatment, as well as pCO2. We have found a significant inverse correlation between delta[HCO3-] pre and post treatment and basal [HCO3-] (r = -0.88; p less than 0.001), with an excellent correction of uremic acidosis, avoiding post dialytic alkalosis. Our long term experience on 6 patients followed up to 31 months, allows us to consider AFB as a feasible and safe treatment, offering a good correction of uremic acidosis and an excellent hemodynamic tolerance. We propose AFB as a competitive technique with respect to bicarbonate hemodialysis, hemodiafiltration and standard biofiltration.