Abstract: A catheter device for accessing an internal body site along a small-diameter vessel path containing branch points in which the path may follow either the larger of smaller-diameter branch vessels. The device includes a catheter having a distal-end inflatable ballon and a distal-end aperture. A guide wire in the device is designed to block the aperture at one or more axial wire positions, allowing the balloon to be inflated by fluid infusion into the catheter. In a catheter placement operation, the catheter is advanced along the vessel path toward the selected target site. When the leading end of the catheter encounters a branch point at which the path follows the larger-diameter vessel, the guide wire is moved to a position which blocks the catheter aperture, and fluid is infused into the catheter to inflate the balloon. The catheter is then carried by hydrodynamic flow into the larger-diameter vessel.