Blood volume was measured continuously during hemodialysis using the authors' hematocrit continuous measurement system in an attempt to determine any relationship between dialysis-induced hypotension with acute onset and blood volume. Hypotension resulted approximately 3 h into dialysis in the present study. This point in time was that at which blood volume was more than 5% above the minimum level during the dialysis treatment, and in some patients it was more than 10% over the minimum level of blood volume. At the time dialysis-induced hypotension occurred or before, there was no sharp decrease in blood volume nor any change in the plasma refilling rate. This suggested that this hypotension is caused by a sudden breakdown of the blood pressure support mechanism compensating for decreased blood volume.