Hemofiltration efficacy was assessed in 14 patients with acute renal insufficiency. Purification quality was controlled by the balance of urea as a marker of low molecular mass substances. Ten patients survived. Adequate purification could be attained with the help of hemofiltration disregarding the level of azotemia and catabolism. An indispensible condition for effective treatment was the substitution of an adequate volume of body fluid (approximately 1.2 volume). The effectiveness of the method could be enhanced as many patients tolerated filtration at rate of 200 ml/min and more. A substitution solution was adequately saturated with urea and purification efficacy corresponded to the rate of filtration. In prolonged anuria filtration procedures not less than 2 times a week were considered optimum.