An intravascular balloon occlusion device according to the invention is shown. The device is ideally suited for use in a coronary artery bypass graft procedure. The device includes a body having at least one selectively inflated balloon provided on the distal end thereof. Preferably, the body is a closed end body so that fluid can only flow from the proximal end of the body into the balloon. In use, the distal end of the body and the balloon are inserted into an aperture provided in the aorta. The balloon is inflated and then the device is retracted until the balloon seats against the incision or aperture in the aorta, thereby effectively sealing the aperture from the blood flow through the aorta, but not occluding blood flow through the body of the aorta itself. Next, the graft vessel is telescopically positioned on the occlusion device and mounted to the aorta. Once the vessel is secured thereto, the balloon is deflated and then the occlusion device is retracted from both the aorta and the graft vessel. Finally, the second end of the graft vessel is mounted to the appropriate coronary artery.