A device and method for monitoring an access to a patient for an extracorporeal blood treatment apparatus with an extracorporeal blood circuit are described, as well as an extracorporeal blood treatment apparatus with a device for monitoring the vascular access. The device and method are based on the monitoring of a characteristic property of the blood, in particular the concentration of haemoglobin in the blood flowing in the arterial blood line of extracorporeal blood circuit I of an extracorporeal blood treatment apparatus A. In the event of an incorrect vascular access, the flow conditions change in the communicating intra- and extracorporeal blood circulation system. These changes in the flow conditions can be detected as a change in the haemoglobin concentration. A disconnection of venous puncture cannula for the patient access is ascertained by a reduction in the haemoglobin concentration in the blood in arterial blood line.
