An extracorporeal blood treatment apparatus (1) and a method for checking the connection of a soft bag (30, 24 33, 34) in the apparatus (1). The apparatus (1) comprises a blood treatment device (2), an extracorporeal blood circuit (3, 5) and a fluid circuit (8, 12, 15, 17, 18, 22 41, 42, 44). A control unit (32) is configured to check the connection of a soft bag (30, 24 33, 34) to the extracorporeal blood circuit (3, 5) or to the fluid circuit (22 41, 42, 44) through the following procedure: sucking a medium from a connecting zone (29) through a blood pump (6) or a fluid pump (23 37, 39) of the apparatus (1) measuring at least a pressure trend (P1, P1 - P2, P2 - P1, Pwdr, Pwdr - Pret, Pret - Pwdr) over time in the extracorporeal blood circuit (3, 5) or in the fluid circuit (22 41, 42, 44) through at least a pressure sensor (25, 26) establishing from said measured pressure trend (P1, P1 - P2, P2 - P1, Pwdr, Pwdr - Pret, Pret - Pwdr) if the soft bag (30, 24 33, 34) is connected to the extracorporeal blood circuit (3, 5) or to the fluid circuit (22 41, 42, 44) at the connecting zone (29).
