Background: We intend to know whether time differences between regular-hour visitors and off-hour visitors in symptom-to-door time or door-to-balloon time influence clinical outcomes after percutaneous coronary intervention (PCI) in ST-segment elevation myocardial infarction (STEMI). Methods: In the patients registered in the Korean Acute Myocardial Infarction Registry (KAMIR) between November 2005 and November 2006, we included the patients those who arrived at medical center within 12 hours after onset of chest pain and underwent PCI under the diagnosis of STEMI. The subjects divided into two groups according to their visiting time (regular-hours: weekdays 7:00 am to 5:00 pm, off-hours: weekdays 5:00 pm to 7:00 am and weekends). We analyzed in-hospital mortality, 1, 6, 12-month major adverse cardiac events (MACE), all cause mortality, and the composite of cumulative mortality and MACE among 1985 patients (regular hours: 1000 patients, off-hours: 985 patients) who were satisfied with criteria for enrollment according to their visiting time. Results: Door-to-balloon times were longer during off-hours (103.2짹55.5 minutes) than regular hours (92.2짹55.0 minutes, p<0.001). In contrast, symptom-to-door times were longer during regular hours (207.3짹153.7 minutes) than off-hours (186.9짹146.6 minutes, p=0.002). In-hospital mortality (regular hours: 4.7 %, off-hours: 4.5 %, p=0.831) and overall mortality in one-year clinical follow-up (regular hours: 14.2 %, off-hours: 11.3 %, p=0.187), there were no differences between two groups. Also, MACE in one-month (regular hours: 2.4 %, off-hours 2.2 %, p=1.0), in 6-month (regular hours: 9.0 %, off-hours: 9.2 %, p=0.926), and in 12-month (regular hours: 15.7 %, off-hours: 15.2 %, p=0.858) did not show any differences. Furthermore, there was no differences between two groups in the composite of cumulative mortality and MACE in one-year (regular hours: 23.6 %, off-hours: 22.0 %, p= 0.555). Conclusions: Patients visited medical center more earlier after onset of symptom in off-hours, but associated with longer times to treatment for PCI. Clinical effectiveness of regular hours and off-hours PCI could be equivalent in Korea.