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In advanced congestive heart failure with fluid retention, extracorporeal ultrafiltration (UF) causes persistent relief of edema or anasarca through hemodynamic and humoral changes that interrupt refractoriness to diuretics. The intra and extravascular fluid partition in congestive heart failure, as well as changes occurring in the two compartments following fluid withdrawal with UF, are unknown. In 8 congestive heart failure patients with severe fluid retention undergoing UF, we measured total (TBV), intrathoracic (ITBV) and pulmonary blood volumes (PBV), and extravascular lung water (EVLW). The intra and extravascular volumes were evaluated by a fiberoptic thermal dye dilution monitoring system, before, at the end of UF (3697 +/- 699 ml) and 24 hours later. Baseline data were compared with those of 10 subjects without heart failure undergoing coronary bypass surgery. In congestive heart failure patients, as compared with controls, TBV was normal, the intrathoracic blood content (ITBV, PBV and PBV/TBV ratio) was increased and EVLW was normal. UF did not induce significant changes in TBV and in EVLW, and reduced ITBV, PBV and PBV/TBV ratio, suggesting that a shift of fluid from the intra to the extrathoracic intravascular compartment occurred. Because both TBV and EVLW were not affected by the procedure, the largest proportion of fluid removed by UF derived from the systemic extravascular space. Both pulmonary wedge and right atrial pressures significantly decreased after UF, and cardiac output increased. In conclusion, congestive heart failure is associated with normal TBV and EVLW content and with intravascular intrathoracic hypervolemia and extrathoracic hypovolemia. UF induces hemodynamic improvement through a selective fluid removal from the extravascular systemic space without changes in both TBV and EVLW.