Recent advances in the surgery of the sympathetic nervous system have made it increasingly necessary to develop a simple method for determining early in the course of peripheral vascular disease whether the diminished flow of blood to an extremity is due to simple arterial spasm or to obliterative structural disease of the blood vessels. The several tests now in use, though different in method, are similar in principle. The surface temperature of the distal portion of the cool, exposed extremity is measured thermo-electrically. Dilatation of the peripheral vessels is then produced, and the rise in surface temperature is recorded. The level to which the temperature rises with complete vasodilatation has been determined in persons with normal peripheral circulation. If the surface temperature fails to reach this normal level in a room at suitable temperature, the arteries supplying the part are regarded as being unable to dilate owing to organic changes