A catheter assembly for treatment of a vessel bifurcation. The catheter assembly is adapted for use with a single guidewire, wherein the guidewire is used to both position the catheter assembly within a main vessel of the vessel bifurcated and then be advanced into a branch vessel of the vessel bifurcated. A guidewire lumen of the catheter assembly is defined by a portion of a catheter shaft of the catheter assembly and a portion of a side catheter branch of the catheter assembly. The side catheter branch extends from a distal end of the catheter shaft at a proximal location and is connected at a distal location to a main catheter branch. After the catheter assembly has been advanced over the guidewire to a position within the main vessel at the vessel bifurcation, the guidewire is retracted, a portion of the side catheter branch is advanced into the branch vessel, and the guidewire is advanced into the branch vessel.
