Dialysis-induced hypovolemia occurs because the rate of extracorporeal ultrafiltration exceeds the rate of refilling of the blood compartment. The purpose of this study was to evaluate a method for calculating circulating blood volume (BV) during hemodialysis (HD) from changes in hematocrit (Hct) shortly (2 to 10 min) before and after ultrafiltration (UF) was abruptly stopped. Hct was monitored continuously during 93 HD treatment sessions in 16 patients by an optical technique and at selected times by centrifugation of blood samples. Total plasma protein and albumin concentrations were also measured at selected times. Continuously monitored Hct correlated with Hct determined by centrifugation (R = 0.89, N = 579). Relative changes in BV determined by continuously monitored Hct were not different from those determined by total plasma protein concentration (P = 0.05; N = 273). Calculated BV at the start of dialysis (4.1 +/- 1.3 L) was not different (P = 0.18, N = 12) from that derived anthropometrically from the patient's dry weight (4.6 +/- 0.8 L), and calculated BV when UF was stopped was 3.2 +/- 0.5 L (46 +/- 7 ml/kg body wt). These latter estimates of BV are consistent with those determined previously by dilution techniques in HD patients. It was concluded that (1) relative changes in BV assessed by continuously monitored Hct were unbiased and (2) BV can be determined noninvasively during HD by continuously monitoring Hct and temporarily stopping UF.