BACKGROUND: Dual antiplatelet therapy with a potent P2Y12 inhibitor coupled with aspirin for 1 year is the recommended treatment for patients with acute coronary syndrome (ACS) undergoing percutaneous coronary intervention (PCI). As an alternative, monotherapy with a P2Y12 inhibitor after a short period of dual antiplatelet therapy has emerged as a bleeding reduction strategy. METHODS: We pooled individual patient data from randomized trials that included patients with ACS undergoing PCI treated with an initial 3-month course of dual antiplatelet therapy followed by ticagrelor monotherapy versus continued ticagrelor plus aspirin. Patients sustaining a major ischemic or bleeding event in the first 3 months after PCI were excluded from analysis. The primary outcome was Bleeding Academic Research Consortium type 3 or 5 bleeding occurring between 3 and 12 months after index PCI. The key secondary end point was the composite of death, myocardial infarction, or stroke. Hazard ratios and 95% CIs were generated using Cox regression with a one-stage approach in the intention-to-treat population. RESULTS: The pooled cohort (n=7529) had a mean age of 62.8 years, 23.2% were female, and 55% presented with biomarker-positive ACS. Between 3 and 12 months, ticagrelor monotherapy significantly reduced Bleeding Academic Research Consortium 3 or 5 bleeding compared with ticagrelor plus aspirin (0.8% versus 2.1%; hazard ratio, 0.37 [95% CI, 0.24-0.56]; P<0.001). Rates of all-cause death, myocardial infarction, or stroke were not significantly different between groups (2.4% versus 2.7%; hazard ratio, 0.91 [95% CI, 0.68-1.21]; P=0.515). Findings were unchanged among patients presenting with biomarker-positive ACS. CONCLUSIONS: Among patients with ACS undergoing PCI who have completed a 3-month course of dual antiplatelet therapy, discontinuation of aspirin followed by ticagrelor monotherapy significantly reduced major bleeding without incremental ischemic risk compared with ticagrelor plus aspirin. REGISTRATION: URL: https://www.crd.york.ac.uk/prospero; Unique identifier: CRD42023449646.