Experimental measurements have been made of the coupling efficiency of helical coil applicators operating under conditions simulating the regional hyperthermic heating of human extremities. We have found that for both saline and layered fat-muscle arm-size phantoms, the coupling efficiency ranged from 56 to 86 percent depending upon the specific coil-phantom combination employed. Helical coil applicators, therefore, seem to be inherently efficient devices for extremity heating.