BACKGROUND AND PURPOSE: Severe atherosclerosis in the aortic arch is associated with a high risk of recurrent vascular events, but the optimal antithrombotic strategy is unclear. METHODS: This prospective randomized controlled, open-labeled trial, with blinded end point evaluation (PROBE design) tested superiority of aspirin 75 to 150 mg/d plus clopidogrel 75 mg/d (A+C) over warfarin therapy (international normalized ratio 2-3) in patients with ischemic stroke, transient ischemic attack, or peripheral embolism with plaque in the thoracic aorta>4 mm and no other identified embolic source. The primary end point included cerebral infarction, myocardial infarction, peripheral embolism, vascular death, or intracranial hemorrhage. Follow-up visits occurred at 1 month and then every 4 months post randomization. RESULTS: The trial was stopped after 349 patients were randomized during a period of 8 years and 3 months. After a median follow-up of 3.4 years, the primary end point occurred in 7.6 (13/172) and 11.3 (20/177) of patients on A+C and on warfarin, respectively (log-rank, P=0.2). The adjusted hazard ratio was 0.76 (95 confidence interval, 0.36-1.61; P=0.5). Major hemorrhages including intracranial hemorrhages occurred in 4 and 6 patients in the A+C and warfarin groups, respectively. Vascular deaths occurred in 0 patients in A+C arm compared with 6 (3.4 ) patients in the warfarin arm (log-rank, P=0.013). Time in therapeutic range (67 of the time for international normalized ratio 2-3) analysis by tertiles showed no significant differences across groups. CONCLUSIONS: Because of lack of power, this trial was inconclusive and results should be taken as hypothesis generating. CLINICAL TRIAL REGISTRATION URL: http://www.clinicaltrials.gov. Unique identifier: NCT00235248.