Aims: It is unclear how serial high-sensitivity troponin-I (hsTnI) concentrations affect long-term prognosis in individuals with suspected acute coronary syndrome (ACS)., Methods and Results: Subjects who underwent two hsTnI measurements (Siemens TnI Flex® Reagent) separated by 1-7 h, during a first-time hospitalization for myocardial infarction, unstable angina, observation for suspected myocardial infarction, or chest pain from 2012 through 2019, were identified through Danish national registries. Individuals were stratified per their hsTnI concentration pattern (normal, rising, persistently elevated, or falling) and the magnitude of hsTnI concentration change (<20%, >20-50%, or >50% in either direction). We calculated absolute and relative mortality risks standardized to the distributions of risk factors for the entire study population. A total of 20 609 individuals were included of whom 2.3% had died at 30 days, and an additional 4.7% had died at 365 days. The standardized risk of death was highest among persons with a persistently elevated hsTnI concentration (0-30 days: 8.0%, 31-365 days: 11.1%) and lowest among those with two normal hsTnI concentrations (0-30 days: 0.5%, 31-365 days: 2.6%). In neither case did relative hsTnI concentration changes between measurements clearly affect mortality risk. Among persons with a rising hsTnI concentration pattern, 30-day mortality was higher in subjects with a >50% rise compared with those with a less pronounced rise (2.2% vs. <0.1%)., Conclusion: Among individuals with suspected ACS, those with a persistently elevated hsTnI concentration consistently had the highest risk of death. In subjects with two normal hsTnI concentrations, mortality was very low and not affected by the magnitude of change between measurements., Competing Interests: Conflict of interest: M.P. discloses the following relationships—advisory board: AstraZeneca, Janssen-Cilag, and Novo Nordisk; grant support: Danish Cardiovascular Academy funded by the Novo Nordisk Foundation and the Danish Heart Foundation (grant number CPD5Y-2022004-HF); and speaker honorarium: AstraZeneca, Bayer, Boehringer Ingelheim, and Janssen-Cilag. M.V. has received research grant support or served on advisory boards for American Regent, Amgen, AstraZeneca, Bayer AG, Baxter Healthcare, Boehringer Ingelheim, Cytokinetics, Lexicon Pharmaceuticals, Relypsa, and Roche Diagnostics; speaker engagements with Novartis and Roche Diagnostics; and participates on clinical endpoint committees for studies sponsored by Galmed and Novartis. C.B. discloses the following relationships—speaker honorarium: Bayer. T.B.-S. discloses the following relationships—steering committee member of the Amgen-financed GALACTIC-HF trial, chief investigator and steering committee chair of the Sanofi Pasteur–financed ‘NUDGE-FLU’ trial, chief investigator and steering committee chair of the Sanofi Pasteur–financed ‘DANFLU-1’ trial, chief investigator and steering committee chair of the Sanofi Pasteur–financed ‘DANFLU-2’ trial, and steering committee member of ‘LUX-Dx TRENDS Evaluates Diagnostics Sensors in Heart Failure Patients Receiving Boston Scientific’s Investigational ICM System’ trial; advisory board: Sanofi Pasteur, Amgen, and GSK; speaker honorarium: Novartis, Sanofi Pasteur, and GSK; and research grants: GE Healthcare and Sanofi Pasteur. M.M. is supported by a grant from the Novo Nordisk Foundation (grant number NNFOC0074083) and has received lecture and advisory board fees from Novo Nordisk. C.P.M. is supported by a grant from the National Institutes of Health (K23HL167659) and has received consulting fees/honorarium from Roche Diagnostics and Abbott Laboratories. L.K. discloses the following relationships—speaker honorarium from Novo Nordisk, Novartis, AstraZeneca, Boehringer, and Bayer. N.T.O. discloses the following relationships—research funding: Abbott and Shockwave. D.L.B. discloses the following relationships—advisory board: Angiowave, Bayer, Boehringer Ingelheim, Cardax, CellProthera, Cereno Scientific, Elsevier Practice Update Cardiology, High Enroll, Janssen, Level Ex, McKinsey, Medscape Cardiology, Merck, MyoKardia, NirvaMed, Novo Nordisk, PhaseBio, PLx Pharma, and Stasys; board of directors: American Heart Association New York City, Angiowave (stock options), Bristol Myers Squibb (stock), DRS.LINQ (stock options), and High Enroll (stock); consultant: Broadview Ventures and Hims; data monitoring committees: Acesion Pharma, Assistance Publique-Hôpitaux de Paris, Baim Institute for Clinical Research (formerly Harvard Clinical Research Institute, for the PORTICO trial, funded by St. Jude Medical, now Abbott), Boston Scientific (chair, PEITHO trial), Cleveland Clinic, Contego Medical (chair, PERFORMANCE 2), Duke Clinical Research Institute, Mayo Clinic, Mount Sinai School of Medicine (for the ENVISAGE trial, funded by Daiichi Sankyo; for the ABILITY-DM trial, funded by Concept Medical), Novartis, Population Health Research Institute, and Rutgers University (for the NIH-funded MINT Trial); honoraria: American College of Cardiology (senior associate editor, Clinical Trials and News, ACC.org; chair, ACC Accreditation Oversight Committee), Arnold and Porter law firm (work related to Sanofi/Bristol-Myers Squibb clopidogrel litigation), Baim Institute for Clinical Research (formerly Harvard Clinical Research Institute; RE-DUAL PCI clinical trial steering committee funded by Boehringer Ingelheim; AEGIS-II executive committee funded by CSL Behring), Belvoir Publications (editor in chief, Harvard Heart Letter), Canadian Medical and Surgical Knowledge Translation Research Group (clinical trial steering committees), CSL Behring (AHA lecture), Cowen and Company, Duke Clinical Research Institute (clinical trial steering committees, including for the PRONOUNCE trial, funded by Ferring Pharmaceuticals), HMP Global (editor in chief, Journal of Invasive Cardiology), Journal of the American College of Cardiology (guest editor, associate editor), K2P (co-chair, interdisciplinary curriculum), Level Ex, Medtelligence/ReachMD (CME steering committees), MJH Life Sciences, Oakstone CME (course director, Comprehensive Review of Interventional Cardiology), Piper Sandler, Population Health Research Institute (for the COMPASS operations committee, publications committee, steering committee, and US national co-leader, funded by Bayer), WebMD (CME steering committees), and Wiley (steering committee); other: Clinical Cardiology (deputy editor); patent: Sotagliflozin (named on a patent for sotagliflozin assigned to Brigham and Women’s Hospital who assigned to Lexicon; neither I nor Brigham and Women’s Hospital receive any income from this patent); research funding: Abbott, Acesion Pharma, Afimmune, Aker Biomarine, Alnylam, Amarin, Amgen, AstraZeneca, Bayer, Beren, Boehringer Ingelheim, Boston Scientific, Bristol-Myers Squibb, Cardax, CellProthera, Cereno Scientific, Chiesi, CinCor, Cleerly, CSL Behring, Eisai, Ethicon, Faraday Pharmaceuticals, Ferring Pharmaceuticals, Forest Laboratories, Fractyl, Garmin, HLS Therapeutics, Idorsia, Ironwood, Ischemix, Janssen, Javelin, Lexicon, Lilly, Medtronic, Merck, Moderna, MyoKardia, NirvaMed, Novartis, Novo Nordisk, Otsuka, Owkin, Pfizer, PhaseBio, PLx Pharma, Recardio, Regeneron, Reid Hoffman Foundation, Roche, Sanofi, Stasys, Synaptic, The Medicines Company, Youngene, and 89Bio; royalties: Elsevier (editor, Braunwald’s Heart Disease); site co-investigator: Abbott, Biotronik, Boston Scientific, CSI, Endotronix, St. Jude Medical (now Abbott), Philips, SpectraWAVE, Svelte, and Vascular Solutions; trustee: American College of Cardiology; and unfunded research: FlowCo and Takeda. The other authors report no relevant disclosures., (© The Author(s) 2023. Published by Oxford University Press on behalf of the European Society of Cardiology. All rights reserved. For permissions, please e-mail: journals.permissions@oup.com.)