Biofiltration (BF) was performed on 60 patients from 12 dialytic centers in Puglia. The protocol was 9-10.5 hours a week with 1.2 m2 PAN dialyzers. A dialysate with 140 Na+, 2-2.5 K+, 3.5-4 Ca++, 38 mEq/l acetate was used in 49 patients; the acetate was replaced by bicarbonate (35-40 mEq/l) in 11 patients. The same patients were treated for 1 year with standard acetate dialysis (49 patients) and standard bicarbonate dialysis (11 patients). The two protocols were compared on the basis of the clinical state, BUN and serum creatinine, acid-base balance, PTH, anemia, and nerve conduction velocity (NCV). Favourable effects were achieved in 55 patients. Four patients left the program because of progressive hyperhydration. BUN and serum creatinine levels showed a moderate, but insignificant increase. PTH, anemia and NCV did not worsen. BF gave better correction of metabolic acidosis in the patients undergoing acetate dialysis.