Background Patients benefit from antiplatelet therapy after coronary-artery bypass grafting (CABG) for an acute coronary syndrome. Whether the addition of ticagrelor to aspirin, as compared with aspirin alone, further reduces the risk of adverse cardiovascular outcomes is unclear.Methods In this open-label, registry-based, clinical trial conducted at 22 Nordic cardiothoracic surgery centers, we randomly assigned patients in a 1:1 ratio to receive either ticagrelor plus aspirin or aspirin alone for 1 year after CABG for an acute coronary syndrome. The primary outcome was a composite of death, myocardial infarction, stroke, or repeat revascularization, evaluated at 1 year. A key secondary outcome was net adverse clinical events, defined as a primary-outcome event or major bleeding.Results A total of 2201 patients were randomly assigned to receive ticagrelor plus aspirin (1104 patients) or aspirin alone (1097 patients). The mean age of the patients was 66 years, and 14.4% were women. A primary-outcome event occurred in 53 patients (4.8%) in the ticagrelor-plus-aspirin group and 50 (4.6%) in the aspirin-alone group (hazard ratio, 1.06; 95% confidence interval [CI], 0.72 to 1.56; P=0.77). Net adverse clinical events occurred in 9.1% of patients in the ticagrelor-plus-aspirin group and 6.4% in the aspirin-alone group (hazard ratio, 1.45; 95% CI, 1.07 to 1.97). Major bleeding occurred in 4.9% of patients in the ticagrelor-plus-aspirin group and 2.0% in the aspirin-alone group (hazard ratio, 2.50; 95% CI, 1.52 to 4.11).Conclusions Among patients who underwent CABG for an acute coronary syndrome, ticagrelor plus aspirin did not result in a lower incidence of death, myocardial infarction, stroke, or repeat coronary revascularization than aspirin alone at 1 year. (Funded by the Swedish Research Council and others; TACSI ClinicalTrials.gov number, NCT03560310; EudraCT number, 2017-001499-43; EU Clinical Trials number, 2023-508551-40-00.) After coronary bypass for acute coronary syndrome, adding ticagrelor to aspirin did not reduce cardiovascular events but increased major bleeding at 1 year, as compared with aspirin alone.