An apparatus for the extracorporeal treatment of blood with veno-venous access, of the type comprising a circuit defined by a main pump (2) and by one or more conduits through which the blood to be treated passes at a given flow value (VI), said circuit being provided with an oxygenator (4), which performs a treatment on the blood at a first flow value, and of a hemofilter (7), which performs a treatment on the blood at a second flow value, lower than said first flow value. The apparatus is characterized in that: said oxygenator (4) is arranged and acting on a first portion (21) of the blood circuit and that the hemofilter (7) is arranged and acting on a second portion (22) of the blood circuit arranged parallel to the first portion (21)； said second portion (22) is connected to the first portion (21) downstream and upstream of the oxygenator (4), the blood entering the hemofilter (7) being taken downstream of the oxygenator (4) and the blood exiting the hemofilter (7) being conveyed in the first portion (21) upstream of the oxygenator； there being provided means for regulating the flow (6； 60, 61) for determining in said first portion (21) a flow (V3) of a value equal to the sum of the flow value (V2) of the second portion (22) and of the value of the flow (VI) with which the blood is taken from the patient and returned to the same； said first portion (21) and said second portion (22) of the circuit are passed through only by blood.La présente invention concerne un appareil de traitement extracorporel du sang avec accès veino-veineux, du type comprenant un circuit défini par une pompe principale (2) et un ou plusieurs conduits à travers lesquels le sang à traiter passe à une valeur d'écoulement donnée (VI), ledit circuit étant muni d'un oxygénateur (4), qui effectue un traitement sur le sang à une première valeur d'écoulement, et d'un hémofiltre (7), qui effectue un traitement sur le sang à une seconde valeur d'écoulement, inférieure à ladite première valeur d'écou
