The effects of a mean ultrafiltration of 2000 ml on hemodynamics during regular hemodialysis (RD) and during sequential ultrafiltration hemodialysis (SUH) have been compared in ten patients on maintenance dialysis. Each patient was submitted to two dialysis sessions at 2 days' interval. The hemodynamic studies were performed with a thermistor Swan-Ganz catheter. The control values of mean systemic arterial pressure, cardiac output, and heart rate were similar with either RD or SUH. During ultrafiltration without diffusion, there was an immediate prolonged and significant decrease in cardiac and stroke indexes and in pulmonary wedge pressure. The mean systemic arterial pressure remained unchanged as long as the total vascular resistance was significantly increased. During the diffusion period of SUH, total vascular resistance decreased, and seven patients became hypotensive. When ultrafiltration was associated with diffusion during RD, the total vascular resistance remained stable, despite a decrease in cardiac index. This was found to result in severe hypotensive episodes, despite a moderate ultrafiltration. These results suggest that diffusion can induce arterial vasodilation and poor hemodynamic adaptation to ultrafiltration-induced hypovolemia SUH, undertaken under careful medical control, appears to be an excellent procedure to deplete severely overhydrated dialyzed patients, but it should not be used routinely as a substitute for RD.