Background : The time from 18:00 to 07:00 on the next day is the most vulnerable period for early percutaneous coronary artery intervention (PCI) because of the delayed recruitment of out-of-hospital medical staffs. The 24 hour in-hospital residency system of cardiology specialist will allow early decision making for PCI and activation of emergency recruitment system for medical staffs. Methods : Total 142 patients with ST segment elevation acute myocardial infarction visited Dong-A University Regional Cardiovascular Center from 01 June 2009 to 30 June 2011. Of the 142 patients, 60 patients visited emergency department during the vulnerable period. Time parameters including door-to-balloon time and survival outcomes including survival discharge and 6-month survival rates were analyzed retrospectively. Results : Of the 60 patients, 49 (70%) were male and mean age was 61 짹 13 years. There was no significant difference of the patients��� clinical characteristics before (n = 47) vs. after (n = 23) establishment of 24 hour in-hospital residency system of cardiology specialist. Door-to-balloon times during the vulnerable period were 74 짹 24 vs. 58 짹 13 minutes (p = 0.004). Proportions of the patients with door-to-balloon time less than 70 minutes were 52.8 vs. 86.4% (p = 0.009). Survival discharge rates were 93.6 vs. 91.3% (p = 0.724) and 6 month survival rates were 91 and 92% (p = 0.996). Conclusion : The establishment of 24 hour in-hospital residency system of cardiology specialist is beneficial for decreasing door-to-balloon time during the vulnerable period for primary PCI.