A controlled in vitro and in vivo comparative evaluation of four Doppler ultrasonic blood velocimeters was performed to evaluate their ability to measure fluid velocity throughout a wide range of flow rates and velocity profiles, during both steady and pulsatile flow. The instruments tested were the Versatone Bidirectional Doppler M-9, the Sonicaid BV380, the Parks 806 B, and the Delalande DUD 400. All instruments responded linearly to velocities of from 5 to 70 cm/s, and were accurate to within 5% through a wide range of in vitro changes in velocity, pulse frequency, and cross-sectional velocity profile. Quantitative velocity measurements and their derivatives are an inexpensive, simple, and useful tool in the evaluation conditions of patients with peripheral vascular disease, and widespread clinical trials of their value are appropriate at this time.