Side branch protection of bifurcation lesions with guide wires treated with directional coronary atherectomy has been performed safely in a small number of patients using a nickel/titanium (Nitinol) alloy guide wire. There is no information about the safety of using other guide wires in this scenario. The purpose of this study was to microscopically examine a variety of guide wires exposed to the directional coronary atherectomy (DCA) device in vitro. Eight common guide wires used in percutaneous coronary angioplasty were positioned in a side branch of a simulated coronary bifurcation and exposed to the operating DCA device. The guide wires were examined grossly and under scanning electron microscopy. The Nitinol guidewire suffered no discernible damage in comparison to varying degrees of damage seen on the other guide wires, and this guidewire appears to be best suited for side branch protection. Examination of the protecting guide wire and cutting cylinder used in the initial patients also showed no damage. Further clinical trials are necessary to assess the efficacy of DCA under such conditions.