Method for monitoring the operation of a medical device for extracorporeal blood treatment, the device comprising a blood treatment unit having a blood chamber that is traversed by blood and a treatment chamber that is traversed by a blood treatment fluid and separated from the blood chamber by way of a semipermeable membrane, an arterial fluid line that is connected in fluid-conveying fashion at one end to an inlet in the blood chamber and at the other end to an arterial vessel access (Z a) of a patient and that is traversed by an arterial volume flow (Q a), a venous fluid line that is connected in fluid-conveying fashion at one end to an outlet from the blood chamber and at the other end to a venous vessel access (Z v) of the patient and that is traversed by a venous volume flow (Q v), and a blood pump that is configured to convey a pump volume flow (Q p) through an extracorporeal blood circuit comprising the blood chamber, the arterial fluid line and the venous fluid line, wherein the method includes the steps of: ascertaining a control parameter that represents a relationship between the venous volume flow (Q v) through the venous fluid line and the arterial volume flow (Q a) through the arterial fluid line; ascertaining a theoretical control parameter (Kt) that represents a theoretical relationship between the venous volume flow (Q v) and the arterial volume flow (Q a), the theoretical control parameter (Kt) being ascertained as a function of the pump volume flow (Q p) and a filtration volume flow (Q F), which is led away from the blood chamber via the semipermeable membrane; comparing the ascertained control parameter to the ascertained theoretical control parameter (K t); monitoring the operation on the basis of the comparison. a blood treatment unit having a blood chamber that is traversed by blood and a treatment chamber that is traversed by a blood treatment fluid and separated from the blood chamber by way of a semipermeable membrane, an arterial fluid line that is connected in fluid-conveying fashion at one end to an inlet in the blood chamber and at the other end to an arterial vessel access (Z a) of a patient and that is traversed by an arterial volume flow (Q a), a venous fluid line that is connected in fluid-conveying fashion at one end to an outlet from the blood chamber and at the other end to a venous vessel access (Z v) of the patient and that is traversed by a venous volume flow (Q v), and a blood pump that is configured to convey a pump volume flow (Q p) through an extracorporeal blood circuit comprising the blood chamber, the arterial fluid line and the venous fluid line, wherein the method includes the steps of: ascertaining a control parameter that represents a relationship between the venous volume flow (Q v) through the venous fluid line and the arterial volume flow (Q a) through the arterial fluid line; ascertaining a theoretical control parameter (Kt) that represents a theoretical relationship between the venous volume flow (Q v) and the arterial volume flow (Q a), the theoretical control parameter (Kt) being ascertained as a function of the pump volume flow (Q p) and a filtration volume flow (Q F), which is led away from the blood chamber via the semipermeable membrane; comparing the ascertained control parameter to the ascertained theoretical control parameter (K t); monitoring the operation on the basis of the comparison. ascertaining a control parameter that represents a relationship between the venous volume flow (Q v) through the venous fluid line and the arterial volume flow (Q a) through the arterial fluid line; ascertaining a theoretical control parameter (Kt) that represents a theoretical relationship between the venous volume flow (Q v) and the arterial volume flow (Q a), the theoretical control parameter (Kt) being ascertained as a function of the pump volume flow (Q p) and a filtration volume flow (Q F), which is led away from the blood chamber via the semipermeable membrane; comparing the ascertained control parameter to the ascertained theoretical control parameter (K t); monitoring the operation on the basis of the comparison.