1. Ultrasound echo tracing is a simple, atraumatic method for examining prosthetic valve function. 2. In vitro experiments have elucidated the meaning of traces obtained from various types of caged balls. 3. By the use of this method ball diameter, poppet excursion, and opening and closing velocities can be measured in patients. 4. Both in vitro and in vivo experience has demonstrated that the measurements obtained are accurate and reproducible.