The present invention relates to a device and a method for blood treatment in a single-needle operating mode. The blood treatment device has an extracorporeal blood circuit comprising a blood supply line leading to the inlet of a blood treatment unit and a blood return line leading away from the outlet of the blood treatment unit. Blood collecting means, for example a blood collecting container, is provided in the blood return line, said means being connected to means for storing gas, for example an air reservoir, by a flow line. The flow connection comprises two line branches, one branch being used to transfer gas from the air reservoir into the blood collecting container and the other branch being used to transfer gas from the blood collecting container into the air reservoir. The connection branch for transferring gas from the air store into the blood collecting container contains means for compressing air, for example a compressor. The device according to the present invention may be operated in such a way that the filling level of the means for collecting blood can be determined as precisely as possible.
