During surgical procedures with extracorporeal circulation the method of hemoconcentration by means of blood ultrafiltration was employed to control hemodilution in 84 patients. Disposable plate or hollow fiber dialyzers and a specially designed hemoconcentrator were used. The amount of ultrafiltrate received from one patient varied between 750 and 6250 cc, with a mean of 2588 +/- 221 cc, which was almost similar to the amount of cardioplegic solution given to the patients. Hemoconcentration by means of ultrafiltration is a relatively simple, safe and effective method of hemodilution control during extracorporeal circulation. In contrast to the centrifuging method (plasmapheresis) it preserves plasma proteins. The method is indicated for patients undergoing surgical procedures with heart-lung bypass and uncontrolled profound hemodilution, especially for patients with renal insufficiency. Hemoconcentration helps to reduce blood loss and bank blood usage. The employment of this method is not accompanied by any significant disturbances in humoral homeostasis. Hemoconcentration requires a larger volume of electrolyte solutions (sodium bicarbonate, potassium chloride) during heart-lung bypass. The method can be recommended for routine use in clinics operating on the open heart.