This paper describes the development and practical application of a fringe analysis system for the high-speed measurement of human body shape and position. The application for this system is in the measurement of patient surface location and shape during the delivery of radiotherapy treatment for cancer. The system uses a twin-fiber interferometer as the basis for fringe production. The fringes are then projected onto the surface to be measured and captured by a CCD camera before being analyzed using Fourier-transform fringe analysis by a computer system. The novel features of this work are in the way in which the system has been realized, with maximum robustness and speed as primary goals. This has led to the development of a number of new techniques in data preprocessing, use of the algorithm itself, and calibration. Particular features include the larger than customary field of view, the use of noncollimated fringes, toleration of the damaging radiation environment, robustness to the variable color, texture, and profile of the surface, and the ability to operate at high speed with a conventional PC platform. The system is capable of measuring in excess of 32,000 surface points per second. Further developments of the system are also described, which are intended to extend its capabilities further.