Objectives: Outpatient (OPD)-based percutaneous coronary intervention (PCI) with 1-day stay is convenient and reported to be safe previously. But, there are still concerns about its safety due to disastrous results of acute complications of PCI. So we sought to evaluate the safety of OPD-based PCI by analyzing the data of our prospective 4-year registry. Methods: From 2001 to 2005, among 758 patients of OPD-based PCI registry of our hospital, we enrolled 658 patients who were planned to undergo OPD-based PCI. Patient with unsuccessful PCI result, left main bifurcation lesion, severe heart failure, renal failure and acute myocardial infarction within 1 week were excluded. From telephone surveys and medical records, we evaluated the incidence of procedure-related complications and recurrent chest pain at day 1 and combined endpoint of major adverse cardiac event (MACE) of unexpected hospitalization or emergency room (ER) visit with related problems, death or MI at post-PCI day 7 and 30. Results: After excluding 76 cases as high risk procedures or unsuccessful PCI results, 496 patients were discharged on same day (study group) and 86 were remained in hospital for observation according to the preferences of physician or patient(control group). Mean age of study group was 59.5짹9.8 years. Baseline characteristics were similar in both groups regarding hypertension, stroke, diabetes, clinical diagnosis, number of involved vessels, and lesion characteristics. According to the telephone survey on day 1, 14 patients reported symptoms of chest pain and 25 reported localized pain related with procedures. Among the patients who reported these symptoms on day 1, only 1 revisited ER during first week. During 7 days, 8(1.6%) patients visited ER, 4 were admitted and 2 had undergone repeated PCI due to subacute thrombosis. There were no differences in MACE at day 7 between groups (1.6% vs 1.2%, p=0.66). One patient of study group had died suddenly at 1-month follow-up, but there were still no differences in MACE at 30 days either (3.0% vs 3.5%, p=0.74). Conclusions: OPD-based 1-day PCI is safe and patients can be discharged on the same day without an increased cardiac risk after uncomplicated PCI.