The present invention relates to a method of determining the body temperature or a temperature correlated therewith of a patient connected to an extracorporeal blood circuit, wherein the extracorporeal blood circuit has a heat exchanger which is flowed through by blood at one side and by a heat carrier medium at the other side, wherein the temperature (Tdi) of the heat carrier medium at the inlet side is measured at the inlet of the heat exchanger and the temperature (Tdo) of the heat carrier medium at the outlet side is measured at the outlet of the heat exchanger and the volume flow of the heat carrier medium (Qd) is measured； and in that the temperature (Tbi) of the blood at the inlet side is determined at the inlet of the heat exchanger in accordance with the relationship Tbi=Tdi (Qd/D) (Tdo−；Tdi), where the value D is a value characteristic of the heat transfer by the heat exchanger.
