Background: Systematic electrocardiogram (ECG) monitoring improves detection of covert atrial fibrillation in stroke survivors but the effect on secondary prevention is unknown. We aimed to assess the effect of systematic ECG monitoring of patients in hospital on the rate of oral anticoagulant use after 12 months., Methods: In this investigator-initiated, randomised, open-label, parallel-group multicentre study with masked endpoint adjudication, we recruited patients aged at least 18 years with acute ischaemic stroke or transient ischaemic attack without known atrial fibrillation in 38 certified stroke units in Germany. Patients were randomly assigned (1:1) to usual diagnostic procedures for atrial fibrillation detection (control group) or additional Holter-ECG recording for up to 7 days in hospital (intervention group). Patients were stratified by centre using a random permuted block design. The primary outcome was the proportion of patients on oral anticoagulants at 12 months after the index event in the intention-to-treat population. Secondary outcomes included the number of patients with newly diagnosed atrial fibrillation in hospital and the composite of recurrent stroke, major bleeding, myocardial infarction, or death after 6 months, 12 months, and 24 months. This trial was registered with ClinicalTrials.gov, NCT02204267, and is completed and closed for participants., Findings: Between Dec 9, 2014, and Sept 11, 2017, 3465 patients were randomly assigned, 1735 (50·1%) to the intervention group and 1730 (49·9%) to the control group. Oral anticoagulation status was available in 2920 (84·3%) patients at 12 months (1484 [50·8%] in the intervention group and 1436 [49·2%] in the control group). For the primary outcome, at 12 months, 203 (13·7%) of 1484 patients in the intervention group versus 169 (11·8%) of 1436 in the control group were on oral anticoagulants (odds ratio [OR] 1·2 [95% CI 0·9-1·5]; p=0·13). Atrial fibrillation was newly detected in patients in hospital in 97 (5·8%) of 1714 in the intervention group versus 68 (4·0%) of 1717 in the control group (hazard ratio [HR] 1·4 [95% CI 1·0-2·0]; p=0·024). The composite of cardiovascular outcomes and death did not differ between patients randomly assigned to the intervention group versus the control group at 24 months (232 [13·5%] of 1714 vs 249 [14·5%] of 1717; HR 0·9 [0·8-1·1]; p=0·43). Skin reactions due to study ECG electrodes were reported in 56 (3·3%) patients in the intervention group. All-cause death occured in 73 (4·3%) patients in the intervention group and in 103 (6·0%) patients in the control group (OR 0·7 [0·5-0·9])., Interpretation: Systematic core centrally reviewed ECG monitoring is feasible and increases the detection rate of atrial fibrillation in unselected patients hospitalised with acute ischaemic stroke or transient ischaemic attack, if added to usual diagnostic care in certified German stroke units. However, we found no effect of systematic ECG monitoring on the rate of oral anticoagulant use after 12 months and further efforts are needed to improve secondary stroke prevention., Funding: Bayer Vital., Translation: For the German translation of the abstract see Supplementary Materials section., Competing Interests: Declaration of interests KGH reports speakers' honoraria, consulting fees, lecture honoraria, and study grants from Abbott, AstraZeneca, Bayer Healthcare, Sanofi, Boehringer Ingelheim, Daiichi Sankyo, Pfizer, Bristol-Myers Squibb, Biotronik, Medtronic, Portola, Getemed, Premier Research, WL Gore and Associates, and Edwards Lifesciences. PK receives research support for basic, translational, and clinical research projects from the EU, British Heart Foundation, Leducq Foundation, the UK Medical Research Council, and German Centre for Cardiovascular Research, from several drug and device companies active in atrial fibrillation, and has received honoraria from several such companies in the past. PK is listed as inventor on two patents held by the University of Birmingham, Birmingham, UK (Atrial Fibrillation Therapy WO 2015140571, Markers for Atrial Fibrillation WO 2016012783). RW reports grants from Deutsches Zentrum für Herz-/Kreislaufforschung, during the conduct of the study; personal fees from Bayer, Berlin Chemie, Bristol-Myers-Squibb, CVRx, Daiichi Sankyo, Gilead, Novartis, Pfizer, and Servier; grants and personal fees from Boehringer Ingelheim and Medtronic; and grants from Bundesministerium für Bildung und Forschung, Deutsche Forschungsgemeinschaft, and the EU, outside the submitted work. AK reports speakers' honoraria, consulting fees, and lecture honoraria from Bayer Healthcare, Sanofi, Boehringer Ingelheim, Daiichi Sankyo, Pfizer, Bristol-Myers Squibb, and Medtronic. TR received consulting honoraria, speakers' honoraria and travel support from Bristol-Myers Squibb–Pfizer, Boehringer Ingelheim, Bayer HealthCare, and Daiichi Sankyo, outside the submitted work. KG reports personal fees and non-financial support from Bayer, Boehringer Ingelheim, Bristol-Meyers Squibb, Daiichi Sankyo, and Pfizer. GT has received speakers' honoraria or consulting fees from Acandis, Bayer Healthcare, Boehringer Ingelheim, Covidien, Bristol-Myers Squibb, Portola, Stryker, and Pfizer. DGN has received speakers' honoraria and consulting fees from Bayer, Boehringer Ingelheim, Bristol-Myers Squibb, Daiichi Sankyo, Novartis, and Pfizer. JR has received speakers' honoraria and consulting fees from Bayer, Boehringer Ingelheim, Bristol-Myers Squibb, Pfizer, and AstraZeneca. UL reports honoraria and reimbursements for lectures, participation in studies, scientific cooperations (with Saarland University, Saarland, Germany), consulting, travel, support (of colleagues), or support of scientific meetings by Amgen, Bayer, Boehringer Ingelheim, Daiichi Sankyo, MSD, Sanofi, and Servier, outside the submitted work. RV reports grants, personal fees, and being a shareholder from Bayer; grants from Boehringer; grants and personal fees from Bristol-Myers Squibb and Pfizer; grants from Daiichi Sankyo, Medtronic, and Biogen; personal fees from Javelin, Abbott, and AstraZeneca; and holding shares in Novartis, outside the submitted work. RV is an investigator of Imperial National Institutes of Health Research Biomedical Research Centre and partially funded by the EU's Horizon 2020 research and innovation programme (grant agreement 754517 [PRESTIGE-AF]). PUH reports grants from Charité, Universitätsmedizin Berlin during study conduct (within MonDAFIS for biometry; member of the scientific board); research grants from the German Ministry of Research and Education, German Research Foundation, the Bavarian State (ministry for science and the arts; within STAAB COVID-19), the EU, Berlin Chamber of Physicians, German Parkinson Society, University Hospital Würzburg, Robert Koch Institute, German Heart Foundation, Federal Joint Committee within the Innovationfond, University Hospital Heidelberg (within RASUNOA-prime; supported by an unrestricted research grant to the University Hospital Heidelberg from Bayer, Bristol-Myers Squibb, Boehringer Ingelheim, and Daiichi Sankyo), and University Göttingen (within FIND-AF(randomised); supported by an unrestricted research grant to the University Göttingen from Boehringer Ingelheim), outside the submitted work. ME reports grants from Bayer and fees paid to the Charité from Amgen, Bayer, Boehringer Ingelheim, Bristol-Myers Squibb, Covidien, Daiichi Sankyo, GlaxoSmithKline, Novartis, Pfizer, and Sanofi. All other authors declare no competing interests., (Copyright © 2021 Elsevier Ltd. All rights reserved.)