In a dialysis apparatus the blood pressure is normally monitored at intervals of 5 minutes. According to the invention the current blood pressure curve (33) is measured only during the first 45 minutes at intervals of 5 minutes and then compared with the stored blood pressure curves of previous treatments of the same patient. On the basis of statistical analyses that blood pressure curve is determined from a data storage, which shows the largest similarity to the current blood pressure curve and used as the basic curve (35) for blood pressure control during the current treatment. During further treatment the blood pressure measurement is carried out at intervals from 15 minutes to 30 minutes. This considerably reduces the number of blood pressure measurements and the resultant discomfort of the patient.
