System for extracorporeal blood treatment, which is constructed in two parts and consists of a control module comprising an attached patient module, wherein: the control module has automatic hose clips or controllable valves for fluid lines, a blood pump drive and an evaluation and control electronic system ; the patient module which is a device for extracorporeal blood treatment has a blood circulation system which comprises a venous line , a reservoir , an oxygenator , an air bubble detector , a filter , an arterial line and a blood pump ; the reservoir is equipped with an upper filling level sensor ; the filter comprises a filter element and the filter has a venting line attached to an upper filter part, the venting line being closable by means of a first controllable valve which is arranged in the venting line; the system has a control unit for the fully automatic filling and venting of a patient module according to the invention; an upper face of the filter is higher than the upper filling level sensor of the reservoir and thus partial filling of the filter can be controlled; the control unit, for the purpose of filling and venting the patient module, is designed to close the first controllable valve , which is open after the start of the filling process for the venting line of the filter , after the upper filling level sensor in the reservoir responds in order to control active filling of the filter ; and an air cushion is created in the filter by means of the closed first controllable valve and the thus closed venting line, which air cushion dampens inflowing filling liquid in its flow behaviour such that the filter is only partially filled and an upper region of the filter element remains unwetted by the filling liquid. the control module has automatic hose clips or controllable valves for fluid lines, a blood pump drive and an evaluation and control electronic system ; the patient module which is a device for extracorporeal blood treatment has a blood circulation system which comprises a venous line , a reservoir , an oxygenator , an air bubble detector , a filter , an arterial line and a blood pump ; the reservoir is equipped with an upper filling level sensor ; the filter comprises a filter element and the filter has a venting line attached to an upper filter part, the venting line being closable by means of a first controllable valve which is arranged in the venting line; the system has a control unit for the fully automatic filling and venting of a patient module according to the invention; an upper face of the filter is higher than the upper filling level sensor of the reservoir and thus partial filling of the filter can be controlled; the control unit, for the purpose of filling and venting the patient module, is designed to close the first controllable valve , which is open after the start of the filling process for the venting line of the filter , after the upper filling level sensor in the reservoir responds in order to control active filling of the filter ; and an air cushion is created in the filter by means of the closed first controllable valve and the thus closed venting line, which air cushion dampens inflowing filling liquid in its flow behaviour such that the filter is only partially filled and an upper region of the filter element remains unwetted by the filling liquid. <EOS>