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Monitoring of relative blood volume changes (ΔRBV) has been propagated for the prevention of hemodialysis hypotension. Although the influence of ultrafiltration volume on ΔRBV is well-known, there is no mention in the literature that ΔRBV results should be interpreted differently for the first, second, or third hemodialysis session of the week. To elucidate whether ΔRBV and its derivative, ΔRBV normalized for ultrafiltration volume (ΔRBV/ultrafiltration ratio), vary systematically over the week, we separately analyzed these parameters for the first, second, and third hemodialysis session of the week in 13 chronic hemodialysis patients over a 17-week period. As expected, mean (±SD) ultrafiltration volume was significantly (p < 0.001) higher during the first session than during the second and third hemodialysis sessions (3163 ± 615, 2622 ± 674 and 2607 ± 638 ml, respectively). ΔRBV was significantly (p < 0.01) more negative at the first session than at the second and third hemodialysis sessions (−10.1 ± 2.7, −9.3 ± 3.0 and −9.3 ± 3.1%, respectively). The ΔRBV/ultrafiltration ratio was significantly (p < 0.01) less negative at the first session than at the second and third hemodialysis sessions (−3.2 ± 0.6, −3.5 ± 0.8 and −3.6 ± 0.6%/l, respectively). In conclusion, ΔRBV and the ΔRBV/ultrafiltration ratio differ systematically between the first and other hemodialysis sessions in patients on a thrice-weekly hemodialysis schedule, most likely as a result of different ultrafiltration volumes.