An apparatus for extra-corporeal blood treatment and a method of presetting an initial mode of treatment in the opening phase of an extra-corporeal blood treatment are controlled based on the opening phase of a haemodiafiltration, in which particularly fast removal of electrolytes such as potassium or of urea occurs. The apparatus has a control and computing unit for presetting an initial mode of treatment, in which over a preset interval of time, substituate is fed to the extra-corporeal blood circuit, dialysis fluid is not fed through the dialyser and instead only ultrafiltrate is withdrawn from the dialyser and, on expiry of the preset interval of time, dialysis fluid is fed through the dialyser. Alternatively, dialysis fluid is provided during the preset interval of time through the dialyser at a dialysis fluid flow rate that is smaller than the dialysis fluid flow rate after the preset interval of time has elapsed.
