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Rationale and Objectives Accurate measurement of blood volume flow (in ml/min) is an important clinical goal. This project compared in vitro and in vivo volume flow measurements obtained with a novel, real-time three-dimensional (i.e., four-dimensional) ultrasound scanner (Encore PV; Vuesonix Sensors, Wayne, PA) with those from an invasive transit time flowmeter. Materials and Methods A flow pump was used to generate pulsatile flow rates from 60 to 600 ml/min. The Encore detected absolute blood velocity vectors within a volume. The scanner determined the centerline of the vessel and volume flow was then automatically calculated. Results were compared with those of an invasive technique for volumetric blood flow measurements utilizing a transit-time flowmeter (TS420; Transonic Systems Inc., Ithaca, NY). In vivo, 10 second datasets of the volume flow in the distal aorta of six rabbits were obtained simultaneously with the Encore PV and the flowmeter. Data were compared using linear regression and Bland-Altman analysis (due to the lack of independence). Results In vitro, Encore and flowmeter measurements both matched the flow pump (r2 > 0.99; P < .0001) with mean errors of −11.8% and −0.3%, respectively. Marked underestimation of the true flow rates was encountered with the Encore at the lowest pump setting. In vivo mean volume flows between 10.6 and 79.3 ml/min were measured. Mean and maximum volume flows obtained with the two techniques correlated significantly (P < .0001) with r2 values of 0.86 and 0.62, respectively. The corresponding root-mean-square errors were 6.9% for mean flow and 61.2% for maximum volume flow measurements. Conclusion A new semiautomated four-dimensional Doppler device has been tested in vitro and in vivo. Mean volume flow measurements with this unit are comparable to those of an invasive flowmeter.