It has been suggested that the incidence of hypotensive episodes is less with hemodiafiltration (HDF) than with hemodialysis (HD). The aim of the present study was to assess the BP response during HD and postdilution HDF in relation to the thermal effects of these different treatment modalities by manipulating the dialysate temperature (Td) during HD and the amount of replacement fluid during HDF. In 12 patients, energy transfer rate (in watts) and maximal decline in mean arterial pressure during HD at Td 37.5 degrees C, HD at Td 35.5 degrees C, and postdilution HDF with amounts of replacement fluids infused at room temperature of 1 L/h and 2.5 L/h, respectively, were assessed. All measurements were done twice in each patient. Energy transfer rate was comparable between HD 35.5 degrees C (-26.61 +/- 5.33) and HDF 2.5 L/h (-25.25 +/- 7.91) and was significantly more negative compared with HD 37.5 degrees C (-3.53 +/- 6.44) and HDF 1 L/h (-15.88 +/- 6.94). The maximum decline in mean arterial pressure was significantly higher during HD 37.5 degrees C (-25.6 +/- 13.5) than during HD 35.5 degrees C (-15.1 +/- 13.8) and HDF 2.5 L/h (-19.2 +/- 17.7), whereas there was no significant difference with HDF 1 L/h (-23.0 +/- 14.0). In conclusion, thermal effects during postdilution HDF are dependent on the amount of replacement fluid. Also during HDF, the BP response is strongly related to thermal effects. The use of postdilution HDF with low or intermediate amounts of replacement fluids infused at room temperature seems to have no advantage in preventing hemodynamic instability, compared with HD 35.5 degrees C.