Background: Although acute ST-segment elevation myocardial infarction (STEMI) requires urgent diagnosis and revascularization, which may vary due to the different time of presentation. We evaluated the impact of different presentation time on the management and subsequent outcomes in Korea Acute Myocardial Infarction Registry (KAMIR). Methods: Using KAMIR data collected from November 2005 to September 2007, we analyzed the differences in clinical characteristics and outcomes among 4698 STEMI patients (pts) presented during working hours (weekdays 7 AM to 6 PM) versus off hours (weekends, holidays, and 6 PM to 7 AM weeknights). Results: Overall, 2628 STEMI pts (55.9%) presented during off hours. Compared with pts presented during working hours, off-hour pts were less likely to arrive within 6 hours after symptom onset (59.1% vs 71.5%, P<0.001), less likely to receive primary percutaneous coronary intervention (PCI) (66.9% vs 76.3%, P<0.001) with longer door-to-balloon time [median (h) 1.33 vs 1.00, P<0.001 in primary PCI] and fewer achievements of door-to-balloon time <2 hours (69.4% vs 78.8%, P<0.001 in primary PCI). In addition, pts arriving during off hours were more likely to receive thrombolysis treatment (13.6% vs 4.2%, P<0.001) with longer door-to-needle time [median (h) 0.83 vs 0.47, P<0.001] than those presenting during working hours. However, the pts presented during off hours versus working hours had similar incidence of in-hospital mortality (6.0% vs 5.6%, P=0.172) and 8-month mortality (7.5% vs 6.8%, P=0.334). Multivariate analysis showed that off-hour presentation was not an independent risk factor for in-hospital death [odd ratio (OR) 1.03, 95% confidence interval (CI) 0.78-1.37, P=0.838] or 8-month death (OR 1.11, 95% CI 0.85-1.43, P=0.434). Conclusions: Despite fewer primary PCI and longer door-to-balloon time and door-to-needle time, pts presented with STEMI during off hours showed similar in-hospital and 8-month mortality compared to those of the pts presented during working hours.