Fluid overload is common before, during and after cardiac surgery. The fluid associated with cardiopulmonary bypass (CPB) and cardioplegia is a particularly important source of such fluid overload. In addition, renal dysfunction, which is common in these patients, participates in the pathogenesis of a positive sodium and water balance. Such fluid overload is physiologically undesirable and participates in the pathogenesis of several clinically important complications. Fluid overload can be partly prevented with the use of diuretics. However, in many patients, diuretics do not achieve sufficient sodium and water diuresis. In these patients, the application of hemofiltration (HF) during CPB and also immediately after CPB is an effective and safe approach to the maintenance of fluid homeostasis. If acute renal failure occurs, early intervention with HF may even improve survival.