Background: The optimal dose of aspirin therapy after percutaneous coronary intervention (PCI) remains unclear. We sought to compare the effectiveness and safety of low and high doses of aspirin in preventing adverse outcomes after PCI. Methods: We studied 2,820 consecutive patients who underwent coronary stenting for stable or unstable coronary artery disease (excluding cardiogenic shock) discharged alive without any complications between 2001 and 2007. Patients were categorized based on the discharge aspirin dose into low-dose (81 mg/day, N = 313) or high-dose (162-325 mg/day, N = 2,507) groups. The primary end-points (adjusted using Cox Proportional Hazard and propensity scores) were major adverse cardiovascular events (MACE; a composite of death, myocardial infarction [MI], stent thrombosis [ST], or target vessel revascularization) and net adverse clinical events (NACE; a composite of MACE and thrombolysis in myocardial infarction [TIMI][major or minor] bleeding) at 1 year. Results: In the low-dose versus high-dose groups, MACE occurred in 8.6 versus 9.2% (log rank P = 0.71) and NACE in 11 versus 10% (log rank P = 0.58). In multivariable analysis, low-dose aspirin was not associated with worse outcomes (adjusted HR [95% CI] 0.74 [0.49-1.14] for MACE; 1.03 [0.71-1.50] for NACE). Conclusion: Low-dose aspirin, as prescribed in this study of routine practice, was not associated with worse outcomes compared to high-dose aspirin.