The present invention relates to a dialysis machine having an extracorporeal circuit in which a dialyzer is located which has a chamber on the blood side which is flowed through by blood and a first pressure sensor, which is located upstream of the chamber on the blood side in the direction of flow of the blood, for determining a first pressure value and a second pressure sensor, which is located downstream of the chamber on the blood side in the direction of flow of the blood, for determining a second pressure value, wherein the dialysis machine has first means for determining the pressure difference between the second pressure value and the first pressure value, second means for determining the dynamic viscosity of the blood on the basis of the determined pressure difference, of the blood flow rate through the chamber on the blood side and of one or more characteristic properties of the dialyzer and third means for determining the hematocrit or the hemoglobin value of the blood on the basis of the determined viscosity, and wherein the dialysis machine has a control or regulation unit which is configured such that it sets the blood flow rate and/or the dilution rate and/or the ultrafiltration rate such that the time change of the hematocrit and/or of the hemoglobin value does not exceed a limit value or lies in a desired value range.
