During a difficult coronary angioplasty, the tip of the guide wire became inadvertently wedged in a distal vessel, fractured, and retained in situ. Because of the otherwise successful nature of the angioplasty and the patient's great risk for cardiac surgery, it was elected to leave the wire fragment in place. Recurrence of symptoms led to recatheterization, which in addition to restenosis of the original angioplasty site showed diffuse narrowing of the arterial segment that contained the retained wire fragment. This occurred despite systemic anticoagulation with Coumadin in the interval between procedures. Thus retained guide wire fragments in patent coronary arteries may cause arterial narrowing despite systemic anticoagulation.