Fluid overload may occur in patients with congestive heart failure, especially when there is associated acute renal failure. When the pharmacological approach is not sufficient to maintain the patient’s fluid balance, extracorporeal therapies must be instituted. However, since the ultrafiltration rate may be faster than fluid refilling from the interstitial space, remarkable changes in the circulating blood volume may occur. This may finally result in further worsening of peripheral perfusion due to a significant drop in cardiac output. In order to prevent a fall in the circulating blood volume, slow continuous ultrafiltration (SCUF) should be employed instead of acute intermittent ultrafiltration (UF). To further improve the tolerance to extracorporeal ultrafiltration, the session can be driven by the relative blood volume change monitored on-line with adequate sensors and devices. We utilized one of these systems (Crit-Line, Hemametrics, USA) to compare the relative changes in blood volume during UF and SCUF in 22 patients with fluid overload. Variations in blood pressure were significantly greater with UF than with SCUF even in the presence of similar levels of fluid removal. The variations in blood pressure were paralleled by variations in blood volume, which were greater with UF than with SCUF. In conclusion, extracorporeal ultrafiltration can be used to control the fluid balance in congestive heart failure, but it is advisable to prescribe low ultrafiltration rates over an extended period of time. The use of on-line blood volume monitors can be of further help in improving tolerance and the hemodynamic response.