Hemodialysis vascular access failure represents a major source of morbidity and mortality in chronic hemodialysis (CHD) patients. Serial vascular access blood flow (VABF) measurements are being used as a screening method at an increasing rate. There are limited data on the changes in VABF throughout the hemodialysis session, which may potentially affect the validity of VABF measurement. This study is performed to evaluate the trend in VABF during a given hemodialysis session by serial VABF measurements, along with potential factors that may affect VABF. Thirty-two CHD patients had serial VABF measurements performed during a hemodialysis session. Each patient had three serial VABF measurements during a hemodiaysis treatment (within 30, 90, and 150 minutes from the start of hemodialysis). Mean arterial blood pressure (MAP), ultrafiltration rate, and patient symptoms were recorded simultaneously. The mean VABF was 1,344 +/- 486 mL/min within 30 minutes of hemodialysis and decreased to 1,308 +/- 532 and 1,250 +/- 552 mL/min after 90 and 150 minutes, respectively. This trend was statistically significant (P = 0.03). There was a strong correlation between VABF measurements and MAP, which was more pronounced after 90 minutes of initiation of hemodialysis (r = 0.68; P < 0.001). Using multivariate analysis, it can be predicted that after 90 minutes of hemodialysis, each 10% decrease in MAP would result in an expected decrease of 8% in VABF. There was no effect of type of vascular access, baseline VABF, or amount of ultrafiltration on VABF changes. In conclusion, VABF measurements can be performed up to 2 to 2(1/2) hours from the start of hemodialysis in the majority of patients. The major determinant of VABF changes is MAP. In a subset of patients with a decrease MAP greater than 15%, it is advisable to perform VABF measurement either at the first 90 minutes of hemodialysis or postpone it to another treatment session, when MAP is more stable.