Abstract: A method and apparatus is disclosed for determining the likelihood of success of a bypass graft. One end of the graft is first fixed to the vessel which is to be bypassed. Alternatively, a double lumen balloon tip catheter is inserted into the vessel. A fixed amount of heparinized normal saline is then applied to the proximal end of the graft or catheter and into the vessel. The pressure within the graft vessel is measured and integrated over a given time and the volume of the saline which flows during that time is divided into the integrated pressure to produce a measure of the outflow bed resistance. The proximal side of the vessel is clamped in order to measure distal run-off bed or outflow resistance. A measurement of greater than 1.2 millimeters of mercury per milliliter of injected fluid per minute predicts that the grafts are likely to be unsuccessful.