In patients with severe congestive heart failure (CHF), removal of edema by hemofiltration is associated with significant clinical and hemodynamic improvement, correction of hyponatremia, restoration of urine output and diuretic responsiveness, and with a striking fall in neurohormonal activation. Through these effects, hemofiltration is able to interrupt the progression of CHF toward refractoriness, and to revert the clinical condition of CHF patients to a lower functional class. Fluid refilling from the overhydrated interstitium is the major compensatory mechanism in the prevention of hypovolemia during hemofiltration. Hemofiltration can also be beneficial in patients who have only moderate cardiac insufficiency (NYHA classes II and III) and in whom over-hydration is restricted to the pulmonary district significantly contributing to limiting patients functional capacity. In this setting, hemofiltration, differently from diuretics, is able to remove the increased lung water content and to improve clinical condition, exercise capacity and lung function.