This paper describes a system that has been developed for measuring two-dimensional velocities in real time using ultrasound. The instrument tracks interframe speckle pattern motion using a Sum-Absolute-Difference (SAD) algorithm in order to produce a vector map of 2D velocities. The system's parallel architecture allows calculation of approximately 20,000 vectors per second using the current tracking geometry. A programmable graphics processor encodes individual velocity vectors with color and displays them superimposed on the B-mode image in real time. In vitro tests indicate that the system can track velocities well over the Doppler aliasing limit in any direction in the scan plane with greater than 94% accuracy. A color encoded image obtained from a flow phantom highlights the system's ability to display lateral motion with uniform coloration, in contrast to the two-color display of current ultrasonic Doppler instruments.