Apprehension over the possibility of injury to man by microwaves is based largely on the fact that sufficiently intense and prolonged radiation of this frequency has caused severe injuries to experimental animals. Several hundred workers who have been occupied about radar installations or have been exposed to radar beams have therefore been observed in a comprehensive medical surveillance program. This has been in progress for four years. The initial study was a comparison of 88 nonexposed persons with 226 radar-exposed employees, some of whom had worked with radar as long as 13 years. No acute, transient, or cumulative physiological or pathological changes attributable to microwaves have been revealed by this study in people working with highpower radar transmitters and frequently exposed to their output. These subjects are free to heed the warning sensation of heat and are advised to avoid exposure to any firing beam when in a zone defined by a minimum power density of 0.0131 watts per square centimeter. Since the development of increasingly high-powered transmitters is to be anticipated, the need for more precise and refined statements of human tolerance is evident.