A device for monitoring an extracorporeal blood circuit of an extracorporeal blood treatment device for detection of air bubbles in blood that is conveyed in the extracorporeal circuit by a blood pump, and a method for monitoring an extracorporeal blood circuit for detection of air bubbles, together with a device for monitoring the extracorporeal blood circuit are described. The device and method are based on monitoring the negative pressure in the extracorporeal blood circuit upstream of the blood pump, to allow differentiation between the occurrence of microbubbles which are attributable to cavitation and an entry of air not due to cavitation. When the negative pressure is above a predetermined limit value and microbubbles are detected in the extracorporeal blood circuit, it is deduced that the microbubbles are produced by cavitation and the mechanisms for dissolving the microbubbles are insufficient to remove the microbubbles again before they could reach the patient.