A device and method are used for monitoring an extracorporeal blood treatment device, such as a dialysis machine, which includes an extracorporeal blood circuit having an arterial blood line with an arterial patient port and/or at least one venous blood line with a venous patient port, and a dialysis fluid system which has a dialysis fluid supply line and a dialysis fluid drain line. The monitoring device selects and senses a measured value during operation of the extracorporeal blood treatment device which is suitable for monitoring the blood treatment device to compare a time-related actual course of the measured value with a target course of the measured value stored in a memory, and to determine that there is a defect if, at least in sections, the actual course of the measured value deviates from the target course by more than a defined tolerance.
