In an extracorporeal blood treatment system, non-treated blood may be withdrawn from a patient through an arterial line connected to a first compartment of a blood treatment apparatus. The blood treatment apparatus also includes a second compartment separated from the first component by a semi-permeable membrane. Treated blood may be returned to the patient through a venous line connected to the second compartment. In order to determine blood recirculation, i.e., a mixture of treated blood and non-treated blood in the arterial line, the ultrafiltration rate of plasma water through the semi permeable membrane of the treatment apparatus may be temporarily increased or decreased, and the resulting variation of the blood concentration, i.e., of the ratio of the volume of the plasma volume to the blood volume, is determined. The extent of the mixture of the treated blood and the non-treated blood in the arterial line may then be calculated as a function of the blood flow rate in the treatment apparatus, of the amplitude of the variation of the ultrafiltration rate, and of at least two values of the blood concentration, determined before and during the variation of the ultrafiltration flow rate, respectively.