To monitor an extracorporeal blood circuit during extracorporeal blood treatment, the arterial pressure and venous pressure in the arterial branch and venous branch of the extracorporeal circuit are measured by pressure sensors, and the sum Ps and the difference ΔP of the arterial pressure and/or venous pressure measured at the same time are calculated. A possible error state is indicated when the sum Ps and/or difference ΔP lie outside predetermined limit value windows A and B. To reduce the susceptibility to error, a check is preferably also made to ascertain whether the two above conditions are satisfied for n successive measurements.
