BACKGROUND: Remote ischaemic conditioning with transient ischaemia and reperfusion applied to the arm has been shown to reduce myocardial infarct size in patients with ST-elevation myocardial infarction (STEMI) undergoing primary percutaneous coronary intervention (PPCI). We investigated whether remote ischaemic conditioning could reduce the incidence of cardiac death and hospitalisation for heart failure at 12 months. METHODS: We did an international investigator-initiated, prospective, single-blind, randomised controlled trial (CONDI-2/ERIC-PPCI) at 33 centres across the UK, Denmark, Spain, and Serbia. Patients (age >18 years) with suspected STEMI and who were eligible for PPCI were randomly allocated (1:1, stratified by centre with a permuted block method) to receive standard treatment (including a sham simulated remote ischaemic conditioning intervention at UK sites only) or remote ischaemic conditioning treatment (intermittent ischaemia and reperfusion applied to the arm through four cycles of 5-min inflation and 5-min deflation of an automated cuff device) before PPCI. Investigators responsible for data collection and outcome assessment were masked to treatment allocation. The primary combined endpoint was cardiac death or hospitalisation for heart failure at 12 months in the intention-to-treat population. This trial is registered with ClinicalTrials.gov (NCT02342522) and is completed. FINDINGS: Between Nov 6, 2013, and March 31, 2018, 5401 patients were randomly allocated to either the control group (n=2701) or the remote ischaemic conditioning group (n=2700). After exclusion of patients upon hospital arrival or loss to follow-up, 2569 patients in the control group and 2546 in the intervention group were included in the intention-to-treat analysis. At 12 months post-PPCI, the Kaplan-Meier-estimated frequencies of cardiac death or hospitalisation for heart failure (the primary endpoint) were 220 (8·6%) patients in the control group and 239 (9·4%) in the remote ischaemic conditioning group (hazard ratio 1·10 [95% CI 0·91-1·32], p=0·32 for intervention versus control). No important unexpected adverse events or side effects of remote ischaemic conditioning were observed. INTERPRETATION: Remote ischaemic conditioning does not improve clinical outcomes (cardiac death or hospitalisation for heart failure) at 12 months in patients with STEMI undergoing PPCI. FUNDING: British Heart Foundation, University College London Hospitals/University College London Biomedical Research Centre, Danish Innovation Foundation, Novo Nordisk Foundation, TrygFonden. The ERIC-PPCI trial was funded by a British Heart Foundation clinical study grant (grant number CS/14/3/31002) and a University College London Hospitals/University College London Biomedical Research Centre clinical research grant. The CONDI-2 trial was funded by Danish Innovation Foundation grants (grant numbers 11-108354 and 11-115818), Novo Nordisk Foundation (grant number NNF13OC0007447), and TrygFonden (grant number 109624). DJH was supported by the British Heart Foundation (grant number FS/10/039/28270), the National Institute for Health Research (NIHR) Biomedical Research Centre at University College London Hospitals, the Duke-National University Singapore Medical School, the Singapore Ministry of Health’s National Medical Research Council under its Clinician Scientist-Senior Investigator scheme (grant number NMRC/CSA-SI/0011/2017) and its Collaborative Centre Grant scheme (grant number NMRC/CGAug16C006), and the Singapore Ministry of Education Academic Research Fund Tier 2 (grant number MOE2016-T2-2-021). HEB was supported by the Novo Nordisk Foundation (grant numbers NNF14OC0013337, NNF15OC0016674). RKK is supported by the Oxford NIHR Biomedical Centre. The research was also supported by the NIHR infrastructure at Leeds. The views expressed are those of the author(s) and not necessarily those of the National Health Service, the NIHR, or the Department of Health. This article is based on the work of COST Action EU-CARDIOPROTECTION (CA16225) and supported by COST (European Cooperation in Science and Technology). We thank all study personnel for their invaluable assistance. Sí