Nine patients with intolerance to acetate hemodialysis were treated with biofiltration. It consisted of a 4 hour acetate hemodialysis during which an additional 2 liters of ultrafiltrate was replaced by a bicarbonate solution (100 mEq/l). Hypotensive episodes disappeared and six out of nine patients were symptomless during the session. Compared to standard hemodialysis, arterial blood bicarbonate and pO2 did not drop during biofiltration. Serum acetate levels, which were abnormally high in patients during standard hemodialysis, were reduced during biofiltration to the levels of a control group of acetate tolerant patients. Our data show that positive clinical results are obtained with biofiltration and suggest that they can be due to a better cellular metabolism of acetate induced by the bicarbonate infusion.