The invention relates to a device and a method for monitoring the connection of a blood l treatment unit 1 to fluid system II of an extracorporeal blood treatment apparatus, wherein blood treatment unit 1 comprises an inlet 4a and an outlet 4b for connection to the fluid system and the fluid system comprises a line system with a first line segment 10b which is connected to inlet 4a and a second line segment 12a which is connected to outlet 4b of blood treatment unit Moreover, the invention relates to an extracorporeal blood treatment apparatus with a device for monitoring the connection of the blood treatment unit to the fluid system. The device according to the invention and the method according to the invention for monitoring the connection of a blood treatment unit to the fluid system of an extracorporeal blood treatment apparatus is based on the measurement of the pressure in fluid system II 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.
