On-line monitoring techniques are today available to help the operators in carrying out a dialysis session with less complications and symptoms. Among these devices, the monitor for blood volume measurement during extracorporeal circulation is one of the most effective. We compared in a cross over study, sessions of intermittent haemodialysis and sessions of continuous haemofiltration to evaluate the haemodynamic response to fluid withdrawal and the relevant modifications in circulating blood volume. The superior haemodynamic stability observed in continuous haemofiltration, was supported by a constant circulating blood volume as demonstrated by the monitoring technique. In intermittent haemodialysis, frequent hypotensive episodes were documented in correspondence with a significant decrease of circulating blood volume. This on-line monitoring technique appears to be a simple and effective way to perform a smooth dialysis or haemofiltration session in critically ill patients with acute renal failure.