The present invention relates to a method and a device for monitoring a blood treatment unit, subdivided by a semipermeable membrane into a blood chamber and a dialysis liquid chamber, of an extra-corporeal blood treatment device, which comprises an extra-corporeal blood circuit having an arterial branch leading to the blood chamber of the blood treatment unit, a venous branch, which extends away from the blood chamber, and a dialysis fluid system, in which the dialysis liquid chamber is arranged. The method according to the present invention and the device according to the present invention are based on the idea that the change of the flow resistance of the dialysis machine is determined based on two measurements, wherein an oscillating pressure signal in the extra-corporeal blood circuit or in the dialysis fluid system is measured before and after the change of the substitution rate and/or ultrafiltration rate, with which the substitute is applied to, or ultrafiltrate is removed from, the blood flowing in the blood circulation system. Preferably, the supply of substitute is interrupted temporarily.