A new dialysis method, termed hemodiafiltration, is proposed. The procedure consists of the combination of hemofiltration and conventional hemodialysis using high flux membranes with a transmembrane pressure (TMP) of 300--500 mmHg and a dialysate flow of 900 ml/min. Due to the combination of convective mass transfer and diffusion, the clearance values of both small and larger molecules are significantly higher than during hemofiltration or hemodialysis alone with the same membranes. The removal of excess water is better tolerated than during hemodialysis. With this new method, six patients have been successfully treated for six months (three times per week, three hours per treatment) without side effects. Thus hemodiafiltration appears to be the method of choice to shorten dialysis time.