The temperature of the blood returning from an extracorporeal circuit may influence the vascular stability. Mean arterial blood pressure, heart rate and the temperature in the in-and outlet lines for blood and dialysis fluid of the dialyzer (TB<sub>a</sub>, TB<sub>v</sub>, TD<sub>i</sub> and TD<sub>o</sub>) were continuously measured in 8 patients suffering from vascular instability during standard dialysis. The TB<sub>v</sub> was adjusted to 37, 36 and 35 °C at the start of dialysis by manipulating TDi. The patients were studied two times at each temperature level during a 4-hour dialysis. At the start of dialysis TB<sub>a </sub>was 35.7 ± 0.3°C. The patient’s mean arterial blood pressure and heart rate decreased and increased, respectively, continuously during TB<sub>v</sub> 37 and 36 °C experiments, but were fairly stable at a TB<sub>v</sub> of 35°C. The standard TD<sub>i</sub> of 37 °C should be abandoned for a temperature which is similar to TB<sub>a</sub> (35.7 °C) to avoid the vascular effects of heating or cooling the blood in the extracorporeal system.