Three different Doppler ultrasonic flowmeters were evaluated for quantitative reliability in measuring peripheral arterial blood flow. One was capable of discriminating between forward and retrograde flow and demonstrated excellent linearity in measuring arterial velocity, when compared with an electromagnetic flowmeter. The importance of distinguishing between forward and reverse flow was demonstrated in animal experiments with several pharmacological agents. Seventy-three studies have been performed on patients with vascular disease, both pre-operatively and postoperatively. The direction-sensitive Doppler ultrasonic flowmeter is a rapid, extremely sensitive, noninvasive, and reliable technique for semiquantitative peripheral arterial flow assessment.