The clinical efficiency of biofiltration (BF) was evaluated in six hemodialysis patients with poor clinical tolerance for standard hemodialysis. Three were adults (mean age 34 years, mean body weight 67 kg) and three adolescents (mean age 17 years, mean body weight 38 kg). Mean maintenance hemodialysis time was 90.5 months (range 49-132). BF treatments lasted three hours in all cases, for a total of nine hours weekly, with AN69 S membranes and infusion of 3 liters of HCO3 solution (100 mEq/l for the adults, 80 mEq/l for the adolescents). We recorded intra- and inter-dialytic symptoms daily, hematological values and acid-base status monthly. Multimodality evoked potentials were recorded after 3 and 9 months. Biochemical values reached a steady state 9 months from the beginning of the study, metabolic acidosis was corrected more efficiently in both groups at the end of dialysis, but only in the adult patients, were pre-dialysis plasma bicarbonates within normal limits. A clear drop in the number of episodes of intradialytic hypotension was noticed in both groups, but the adolescent patients' tolerance for dialysis did not improve. In conclusion our data show that in adult patients with poor tolerance BF offers a dependable alternative to standard hemodialysis, and the length of treatment can be reduced.