Background In ST-elevated myocardial infarction (STEMI) patients with multivessel disease, percutaneous coronary intervention (PCI) of the non-infarct-related artery (IRA) is not recommended from the ACC/AHA guidelines. The aim of this study was to compare clinical outcomes of multivessel compared with IRA-only revascularization in patients with undergoing primary PCI for STEMI. Methods Between November 2005 to November 2008, 14,885 patients who admitted with STEMI were registered to Korean Acute Myocardial Infarction Registry (KAMIR) database. Of these patients, 1299 (8.7%) patients with multivessel disease and underwent primary PCI with DES were identified and enrolled in this study. Patients were divided into two groups: treatment of culprit vessel angioplasty-only (COR, n=986); treatment of multivessel including non-IRA (MVR, n=313). The study end-point was major adverse cardiac events (MACE) including death, MI and revascularization at 1 year. Results There was no significant difference in clinical chracterisitc beween two groups. During one year follow-up, 123 (15.2%) patients in the COR group and 40 (14.2%) in the MVR group experienced at least one MACE (Log rank p=0.502). Death, MI and revascularization events were also not different between two groups (Log rank p=0.908, 0.908, 0.540, respectively). In MVR group, complete revascularization was performed in 214 (68.5%) patients. The MACE rate was higher in incompletely revascularized than completely revascularized patients (16.2% vs. 9.7%, p=0.017) Conclusions Although, multivessel angioplasty during primary PCI for STEMI did not reduce MACE compared with culprit vessel-only PCI, complete revascularization was associated with improved cardiac outcomes after multivessel PCI.