Background Percutaneous coronary intervention (PCI)-related myonecrosis is frequent and can affect the long-term prognosis of patients. To our knowledge, ticagrelor has not been evaluated in elective PCI and could reduce periprocedural ischaemic complications compared with clopidogrel, the currently recommended treatment. The aim of the ALPHEUS study was to examine if ticagrelor was superior to clopidogrel in reducing periprocedural myocardial necrosis in stable coronary patients undergoing high-risk elective PCI. Methods The ALPHEUS study, a phase 3b, randomised, open-label trial, was done at 49 hospitals in France and Czech Republic. Patients with stable coronary artery disease were eligible for the study if they had an indication for PCI and at least one high-risk characteristic. Eligible patients were randomly assigned (1:1) to either ticagrelor (180 mg loading dose, 90 mg twice daily thereafter for 30 days) or clopidogrel (300-600 mg loading dose, 75 mg daily thereafter for 30 days) by use of an interactive web response system, and stratified by centre. The primary outcome was a composite of PCI-related type 4 (a or b) myocardial infarction or major myocardial injury and the primary safety outcome was major bleeding, both of which were evaluated within 48 h of PCI (or at hospital discharge if earlier). The primary analysis was based on all events that occurred in the intention-to-treat population. The trial was registered with ClinicalTrials.gov, NCT02617290. Findings Between Jan 9, 2017, and May 28, 2020, 1910 patients were randomly assigned at 49 sites, 956 to the ticagrelor group and 954 to the clopidogrel group. 15 patients were excluded from the ticagrelor group and 12 from the clopidogrel group. At 48 h, the primary outcome was observed in 334 (35%) of 941 patients in the ticagrelor group and 341 (36%) of 942 patients in the clopidogrel group (odds ratio [OR] 0.97, 95% CI 0.80-1.17; p=0.75). The primary safety outcome did not differ between the two groups, but minor bleeding events were more frequently observed with ticagrelor than clopidogrel at 30 days (105 [11%] of 941 patients in the ticagrelor group vs 71 [8%] of 942 patients in the clopidogrel group; OR 1.54, 95% CI 1.12-2.11; p=0.0070). Interpretation Ticagrelor was not superior to clopidogrel in reducing periprocedural myocardial necrosis after elective PCI and did not cause an increase in major bleeding, but did increase the rate of minor bleeding at 30 days. These results support the use of clopidogrel as the standard of care for elective PCI. Copyright (C) 2020 Elsevier Ltd. All rights reserved.