Background: Clinical impact of right coronary artery (RCA) involvement on long-term outcomes of patients undergoing unprotected left main coronary artery (ULMCA) revascularization has not been well known. Methods: Between Jan, 2000 and June, 2006, a total of 2311 patients with ULMCA stenosis were enrolled from 12 academic centers in Korea. Percutaneous coronary intervention (PCI) and coronary artery bypass grafting (CABG) were performed in 1141 patients and 1170 patients, respectively. Results: Patients with RCA disease were significantly older and had more prevalence of diabetes, hypertension, hyperlipidemia, smoking, left ventricular dysfunction (EF<40%), and high Euroscore ( 6) than those without RCA disease. During median follow-up of 37.5 months (24.4-54.8), RCA involvement was significantly associated with increase of adverse clinical events (Table). Conclusions: The presence of RCA stenosis in patients with ULMCA lesion who underwent PCI or CABG was associated with unfavorable long-term outcome compared with those without RCA disease.