The study of postural sway is of special interest because of its applications in clinical medicine. Currently, the most accurate device used to study postural sway is the force platform. It is however costly and nonportable. The authors introduce an alternative system that consists of a computer-interfaced ultrasonic device. Ultrasound is emitted by sonar and bounces back from the subject's body. The amplitude of motion of the center of gravity is calculated and displayed graphically in real time. They, and several researchers, have determined that the mean speed of sway is a parameter that could be used to quantify postural sway. They developed software to filter system noise and calculate the mean speed of sway accurately. They tested the sensitivity and reliability by performing measurements of a fixed distance, and of the anterior-posterior and medical-lateral sways of thirty young healthy subjects, 15 males and 15 females. The error on the mean speed of sway was found to be less than 1%. This is no less accurate than the best sway instruments they know of. The results are consistent with force-platform measurements, and demonstrate the reliability of the system, as a means to study accurately postural control.