An improved capacitance system for blood flow measurement by venous occlusion plethysmography is described. It is shown both theoretically and experimentally, that the change in the capacitance between the surface of the segment and an encircling flexible screen cuff at a fixed distance from the skin is related to the fractional change in volume of the part by a simple constant which can be determined from the initial capacitance. Thus, neither calibration nor measurement of segment volume is needed.