Abstract: An illustrative embodiment of the invention measures blood flow through a in or an artery. A balloon catheter is inserted into the vessel and is inflated until the balloon temporarily stops blood flow in order to measure the diameter of the vessel. The balloon is deflated a bit to reestablish flow through the vessel in the annulus that is formed between the outer surface of the balloon and the adjacent surface of the vessel. Apertures in the catheter on opposite ends of the balloon establish fluid communication with the blood to permit the change in blood pressure to be measured as it flows through the annulus in order to derive the blood flow in milliliters per minute (ml/min) or in some other suitable parameter.