A device and method are provided for controlling the balance of fluids during an extracorporeal blood treatment session, according to which treatment, a certain quantity of liquid is withdrawn from the blood which is compensated for by the injection of a substitute liquid into the extracorporeal circuit. A recirculation circuit is provided and pressure is monitored. A pressure deviation within the recirculation circuit reflects a change in the balance between the quantity of liquid withdrawn from the blood and the quantity of liquid injected into the blood, and injection and/or withdrawal rates are altered to obtain a desired balance. The device and method are especially applicable to haemofiltration, haemodiafiltration and plasmapheresis.