A method of positioning a main stent at a vessel bifurcation includes positioning a main guidewire in the main vessel; and advancing a stent delivery system to a position proximate the bifurcation. The stent delivery system includes a catheter with a flexible side sheath attached thereto and a main stent positioned over the catheter. The flexible side sheath is positioned to pass through the side opening in the main stent. The method also includes advancing a branch guidewire through the flexible side sheath and into the branch vessel; and subsequently, advancing the catheter over the main guidewire while advancing the flexible side sheath over the branch guidewire while viewing relative movement of a marker positioned on the flexible side sheath with respect to at least one marker positioned on the catheter.