In a blood dialysis system including a source of substitution fluid (10) and a blood dialysis machine, a hemodiafiltration system having a first dialyzer (1) including a first blood compartment (4) having a first blood inlet which receives blood to be cleaned and a first blood outlet which discharges partially diafiltered blood and a first dialysate compartment (5) having a first dialysate inlet and a first dialysate outlet, means (8) for mixing the partially diafiltered blood with substitution fluid from the source to obtain a blood/substitution fluid mixture, and a second hemofilter (2) including a second blood compartment (11) having a second blood inlet which receives the blood/substitution fluid mixture and a second blood outlet which discharges filtered blood and a second permeate compartment (12) having a second permeate outlet.