Measurement of colloid osmotic pressure (COP) and hydrostatic pressure within the intravascular space is an important factor in the evaluation of body fluid shifts. This paper describes the development of an oncometercatheter, which allows thein vivomeasurement of COP. The device utilizes multiple lumen catheter technology and standard intensive care equipment for monitoring. The physiologic basis for measurement is described and the catheter function explained. Sensitivity and lack of complications make this catheter invaluable in the laboratory determination of COP. The oncometer may be incorporated in a balloon flotation catheter, which may allow for its eventual use in the monitoring of the critically-ill human.