Objectives: This study sought to assess the value, in terms of sample size and cost, of using the coronary artery calcium (CAC) score to enrich the study population of primary prevention randomized controlled trials (RCTs) with participants at high absolute risk of atherosclerotic cardiovascular disease (ASCVD) events., Background: The feasibility of RCTs assessing the efficacy of novel add-on therapies for primary prevention among high-risk individuals treated with statins may be limited by sample size and cost., Methods: We evaluated 3,075 statin-naive participants from the MESA (Multi-Ethnic Study of Atherosclerosis) with estimated 10-year ASCVD risk of ≥7.5%. CAC of >100, CAC of >400, high sensitivity C-reactive protein levels of >2 and >3 mg/l, ankle-brachial index of <0.9, and triglyceride levels of >175 mg/dl were each evaluated as enrichment criteria on top of estimated ASCVD risk of ≥7.5%, ≥10%, ≥15% and ≥20%. For each criterion, using the observed 5-year incidence of CVD, we projected the incidence of CVD assuming a 28% relative risk reduction with high-intensity statin therapy and after addition of novel therapy with additive relative risk reductions of 15% and 25%. Sample size and cost of a hypothetical primary prevention 5-year RCT of a novel therapy on top of statins versus statins alone were then computed by using the projected incidences. Yearly costs per included participant of $6,000 to $9,000 and of $500/$600 per screened nonparticipant were assumed., Results: CAC of >400, present in 15% to 23% participants, consistently identified the subgroups with highest 5-year incident events and outperformed the other features yielding the smallest projected sample size, ranging 33% to 58% lower than using risk estimations alone for participant selection. CAC of >400 also yielded the lowest projected RCT costs, at least $40 million lower than using risk estimations alone. CAC of >100 showed the second-best performance in most scenarios., Conclusions: High CAC scores used as study entry criteria can improve the efficiency and feasibility of primary prevention RCTs evaluating the incremental efficacy of novel add-on therapies., Competing Interests: Funding Support and Author Disclosures This research was supported by contracts HHSN268201500003I and N01-HC-95159 through N01-HC-95169 from the National Heart, Lung, and Blood Institute and by grants UL1-TR-000040, UL1-TR-001079, UL1-TR-001420, and UL1-TR-001881 from the National Center for Advancing Translational Sciences. Dr. Ray acknowledges support from the National Institute for Health Research (NIHR) Imperial Biomedical Research Centre and Imperial College London is grateful for support from the NIHR Applied Research Collaboration for North West London. Dr. Bhatt has served on the Advisory Board of Cardax, Cereno Scientific, Elsevier Practice Update Cardiology, Medscape Cardiology, PhaseBio, and Regado Biosciences; has served on the Board of Directors for Boston Veterans Affairs Research Institute, Society of Cardiovascular Patient Care, and TobeSoft; has served as chair of the American Heart Association Quality Oversight Committee; has served on the Data Monitoring Committee for the Baim Institute for Clinical Research (formerly Harvard Clinical Research Institute, for the PORTICO trial, funded by St. Jude Medical, now Abbott), Cleveland Clinic (including for the ExCEED trial, funded by Edwards), Duke Clinical Research Institute, Mayo Clinic, Mount Sinai School of Medicine (for the ENVISAGE trial, funded by Daiichi Sankyo), and Population Health Research Institute; has received honoraria from the American College of Cardiology (senior associate editor, Clinical Trials and News, ACC.org; vice chair, ACC Accreditation Committee), 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), 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), Medtelligence/ReachMD (continuing medical education [CME] steering committees), Population Health Research Institute (for the COMPASS operations committee, publications committee, and steering committee and U.S. national coleader, funded by Bayer), Slack Publications (chief medical editor, Cardiology Today’s Intervention), Society of Cardiovascular Patient Care (secretary/treasurer), WebMD (CME steering committees); reports the following roles: Clinical Cardiology (deputy editor), National Cardiovascular Data Registry (NCDR)-ACTION Registry Steering Committee (chair), Veteran Affairs Clinical Assessment, Reporting and Tracking Program (VA CART) Research and Publications Committee (chair); has received research funding from Abbott, Afimmune, Amarin, Amgen, AstraZeneca, Bayer, Boehringer Ingelheim, Bristol Myers Squibb, Chiesi, CSL Behring, Eisai, Ethicon, Ferring Pharmaceuticals, Forest Laboratories, Fractyl, Idorsia, Ironwood, Ischemix, Lilly, Medtronic, PhaseBio, Pfizer, PLx Pharma, Regeneron, Roche, Sanofi Aventis, Synaptic, The Medicines Company; has received royalties from Elsevier (editor, Cardiovascular Intervention: A Companion to Braunwald’s Heart Disease) has served as site coinvestigator for Biotronik, Boston Scientific, CSI, St. Jude Medical (now Abbott), Svelte; has served as trustee for the American College of Cardiology; and has performed unfunded research for FlowCo, Merck, Novo Nordisk, and Takeda. Dr. Ray reports personal fees from Aegerion, grants and personal fees from Amgen, Sanofi/Regeneron, Pfizer, and Merck Sharp & Dohme Corp. (MSD), personal fees from AstraZeneca, Cerenis, Akcea, The Medicines Company, Kowa, Novartis, Cipla, Lilly, Algorithm, Takeda, Boehringer Ingelheim, AbbVie, and Silence Therapeutics. Dr. Reddy’s has received personal fees from Bayer, Daiichi Sankyo, Esperion, AbbVie, Zuelling Pharma, and Resverlogix, outside the submitted work. Dr. Nasir is on the Advisory Boards of Amgen, Novartis, and The Medicine Company; and his research is partly supported by the Jerold B. Katz Academy of Translational Research. All other authors have reported that they have no relationships relevant to the contents of this paper to disclose., (Copyright © 2021 American College of Cardiology Foundation. Published by Elsevier Inc. All rights reserved.)