Device for recirculation measurement by means of thermodilution during extracorporeal blood treatment, having: a temperature change device, which is arranged to heat and/or to cool blood in a venous line segment of the blood line system for an at least almost continuous recirculation measurement; and a detection device which is configured to measure a change of a physical quantity in an arterial line segment of the blood line system, said change resulting from the heating and/or cooling of the blood, wherein the detection device is arranged to detect a first physical quantity in a case free of recirculation in which no recirculation from venous blood into arterial blood occurs during the extracorporeal blood treatment, and to detect a physical quantity changed by the cooling and/or heating of the blood in the venous line segment by the temperature change device as a second physical quantity in the blood line system in a case involving recirculation in which a recirculation of venous blood into arterial blood occurs during the extracorporeal blood treatment; a pump which is arranged to pump blood in a blood line system of an extracorporeal blood circuit, wherein the pump is able to be controlled depending on a quantity change detected at the detection device to set an optimal blood flow for a dialysis treatment, characterised in that the temperature change device is arranged to heat and/or to cool the blood cyclically in the venous line segment; and the device is arranged to determine an occurrence of a recirculation in the case of detection of a first quantity change at the detection device and to determine a disappearance of the recirculation in the case of detection of a second quantity change at the detection device, to combine mathematically a first blood flow in the case of the first quantity change and a second blood flow in the case of the second quantity change into a resulting blood flow and to control the pump to set the resulting blood flow as an optimal blood flow. a temperature change device, which is arranged to heat and/or to cool blood in a venous line segment of the blood line system for an at least almost continuous recirculation measurement; and a detection device which is configured to measure a change of a physical quantity in an arterial line segment of the blood line system, said change resulting from the heating and/or cooling of the blood, wherein the detection device is arranged to detect a first physical quantity in a case free of recirculation in which no recirculation from venous blood into arterial blood occurs during the extracorporeal blood treatment, and to detect a physical quantity changed by the cooling and/or heating of the blood in the venous line segment by the temperature change device as a second physical quantity in the blood line system in a case involving recirculation in which a recirculation of venous blood into arterial blood occurs during the extracorporeal blood treatment; a pump which is arranged to pump blood in a blood line system of an extracorporeal blood circuit, wherein the pump is able to be controlled depending on a quantity change detected at the detection device to set an optimal blood flow for a dialysis treatment, characterised in that the temperature change device is arranged to heat and/or to cool the blood cyclically in the venous line segment; and the device is arranged to determine an occurrence of a recirculation in the case of detection of a first quantity change at the detection device and to determine a disappearance of the recirculation in the case of detection of a second quantity change at the detection device, to combine mathematically a first blood flow in the case of the first quantity change and a second blood flow in the case of the second quantity change into a resulting blood flow and to control the pump to set the resulting blood flow as an optimal blood flow.