Background: The clinical variables in patients with acute coronary syndrome (ACS) admitted on working day versus weekend was not clearly verified in Korea. This study was aimed to evaluate the differences of proportion and clinical outcomes between patients who presented ACS admitted on working day and weekend. Methods: Total 13814 patients was enrolled in KAMIR (Korea Acute Myocardial Infarction Registry) from November 2005 to January 2008. Among those patients, 11762 Patients divided into two groups : Group 1(working day, n=8693, 70.7% male, 63짹76 years) and Group II(weekend, n=3069, 70.6% male, 62.73짹12.73 years). Clinical variables and in-hospital and 1 year mortality were compared with between the two groups. Results: Overall proportions of ACS were STEMI 60.2%, NSTEMI 38.5%. About 26% of patients was admitted during weekend. The proportion of patients with ACS presenting with STEMI is higher on group 2 than group 1 (59.4% vs. 62.5%, p<0.000). The percentage of NSTEMI in ACS is higher on group I compared with group II (39.2% vs. 36.8%, p<0.000)(1.4% vs. 0.7%, p<0.000). Cardiovascular risk factors were not different between two groups. In laboratory findings, NT pro-BNP level was higher in group I than group II. Maximal-CK and CK-MB level were higher in group II than group I. Primary PCI was more frequently used in patients with STEMI of group I than that of group II (77.8% vs. 71.1%, p<0.000). Also early invasive treatment more frequently underwent in patient with NSTEMI on group I than group II (51.4% vs. 43.1%, p<0.000). Group II had significantly higher in-hospital death than group I (5.2% vs. 6.4%, p=0.014). One month death more occurred in group II compared with group I (6.6% vs. 7.9%, p=0.022). In Cox proportional hazard analysis, one-year morality was more higher in group II than group I (9.9% vs. 11.2%, p=0.039; hazard ratio=1.152, (95%���CI 1.007~1.318)). Conclusions: A quarter of patients with AMI visited during weekend. STEMI more developed more frequently during weekend than working day. Long-term mortality was higher in AMI patients presented during weekend.