A device and method for monitoring the connection of a blood treatment unit to a fluid system of an extracorporeal blood treatment apparatus are described, in which the blood treatment unit includes an inlet and an outlet for connection to the fluid system and the fluid system includes a line system with a first line segment connected to the inlet and a second line segment connected to the outlet. An extracorporeal blood treatment apparatus includes a device for monitoring the connection of the blood treatment unit to the fluid system. The device and method for monitoring the connection of the blood treatment unit to the fluid system is based on measurement of pressure in the fluid system of the blood treatment apparatus. Solely on the basis of the pressure measurement, it is ascertained whether the connection of the blood treatment unit to the fluid system is correct or incorrect.
