Background: The effect of presentation time on AMI management and subsequent outcomes has been discussed recently with conflicting results. Little has been known regarding this issue so far in Korea. Methods:Using Korea Acute Myocardial infarction Registry (KAMIR) data collected from November 2005 to September 2007, we analyzed differences in clinical characteristics and outcomes among 7883 AMI 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: Among 7883 AMI pts, 4263 pts (54.1%) arrived during off-hour.Compared with pts presenting during working hours, off-hour pts less frequently presented within 6 hours after AMI onset (43.1% vs 52.2%, P<0.001),received fewer primary precutaneous coronary intervention (PCI) for ST-segment elevation myocardial infarction (STEMI) (66.9% vs 76.3%, P<0.001) and fewer early invasive treatment for Non-ST-segment elevation myocardial infarction (NSTEMI) (44.5% vs 49.7%, P=0.004) with longer door-to-balloon time [median (h), 1.33 vs 1.00, P<0.001 in primary PCI; 18.65 vs 16.39, P<0.001 in early invasive treatment] and fewer had door-to-balloon time <2 hours (69.4% vs 78.8%, P<0.001 in primary PCI; 9.3% vs 19.2%, P<0.001 in early invasive treatment]. In addition, pts off hour pts were more likely to receive thrombolysis (13.6% vs 4.2%, P<0.001) than those pts presented during working hours. Off-hour pts showed numerically higher incidence of in-hospital death (5.2% vs 4.6%, P=0.172) .At 8 months, total death also was significantly higher (7.2% vs 6.0%, P=0.029). Multivariate analysis showed off-hour presentation was not an independent risk factor for in-hospital death [odd ratio (OR) 1.09, 95% confidence interval (CI) 0.86-1.37, P=0.496], but there was a trend for being an independent risk factor for 8-month death (OR 1.20, 95% CI 0.99-1.47, P=0.067). Conclusions: AMI pts presented during off hours are common and associated with substantially longer door-to-balloon time for PCI. Despite similar in-hospital mortality, pts presenting during off hours seem to have more unfavorable mid-term clinical outcomes, suggesting more caution should be exercised for this particular subset of pts.