The invention relates to a device for monitoring an extracorporeal blood circulation of an extracorporeal blood treatment device for detecting air bubbles in the blood which is conveyed in the extracorporeal circulation by a blood pump. Moreover the invention relates to a method for monitoring an extracorporeal blood circulation for detecting air bubbles and to a device for extracorporeal blood treatment using a device for monitoring the extracorporeal blood circulation. The device according to the invention and the method according to the invention are based on monitoring the negative pressure in the extracorporeal blood circulation upstream of the blood pump, in order to be able to distinguish between the formation of microbubbles which are attributable to cavitation and an inclusion of air which is not caused by cavitation. If the negative pressure is above a predetermined limiting value and microbubbles are detected in the extracorporeal blood circulation, it is concluded that the microbubbles are generated by cavitation and the mechanisms for dissolving the microbubbles are not sufficient in order to remove the microbubbles again before they can enter the patient.
