In healthy subjects, the blood volume (BV) increases rapidly after postural change from standing to the supine position. However, little is known about the effect of postural change on BV in long-term hemodialysis (HD) patients. Therefore, we have examined the BV change caused by adopting the supine position from standing by continuous hematocrit monitoring, using the CRIT-LINE instrument, in 8 anuric HD patients. The hematocrit was monitored for 25 min with the patient in the supine position just before HD. The percentage change in the BV (% Δ BV) was calculated from the hematocrit and approximated using the equation: % Δ BV = b – [1 – exp(–c × time (min)] –a × time (min). Coefficient a was the slope of the linear part in the % Δ BV, b was the magnitude of BV increase and c was the rate of BV increase. Then we examined the relationship between the coefficients (a, b and c) and clinical parameters. In all patients, % Δ BV increased quickly after adopting the supine position. The mean increases were 2.8 ± 0.6% after 5 min and 4.8 ± 0.5% after 25 min. There was a significant correlation between the value of % Δ BV calculated from the hematocrit and the value calculated using above equation (0.92 < r < 0.99, p < 0.001). Although coefficient a did not correlate with a clinical parameter, coefficient b showed a significant positive linear correlation with the serum albumin level (r = 0.816, p < 0.05) and coefficient c showed a significant positive linear correlation with the percentage change in interdialytic weight gain (r = 0.736, p < 0.05). Furthermore, based on the % Δ BV, we calculated the change in total BV, which had increased by 181.5 ± 21.9 ml after 25 min in the supine position. In conclusion, the change in the BV with time by continuous hematocrit monitoring using the CRIT-LINE instrument can be approximated by a modified monoexponential equation. BV increased quickly in HD patients after postural change from standing to the supine position.