Decreased hemodialysis access flow is associated with an increased risk of access thrombosis. Duplex ultrasonography can measure access flow and select a subset of patients at increased risk for access failure. With in-line techniques (ultrasound dilution), access flow can be measured during hemodialysis. This study attempted to determine if access flow measured by ultrasound dilution (QA-T) was comparable to that measured by duplex ultrasonography (QA-S). The authors performed 66 simultaneous measurements of hemodialysis access flow in 19 patients by ultrasound dilution and duplex ultrasound with time-domain correlation during 19 hemodialysis treatments. The mean access flow was 1,086 +/- 505 ml/min by ultrasound dilution and 1,026 +/- 513 ml/min with duplex ultrasonography (NS). Regression analysis revealed a linear relationship between the two techniques described by the equation QAT = 246.14 + 0.8104(QAS) (correlation coefficient of 0.83; p < 0.0001). Measurement of hemodialysis access flow by ultrasound dilution was essentially equivalent to that obtained by duplex ultrasound. Additional studies are needed to determine if regular in-line flow measurements can predict and prevent future access thrombosis and decreased the cost of access management.