INTRODUCTION: While screening uptake is variable, many individuals feel they 'ought' to participate in screening programmes to aid the detection of conditions amenable to early treatment. Those not taking part in screening are often presented as either hindered by practical or social barriers or personally at fault. Why some people choose not to participate receives less consideration. METHODS: We explored screening nonparticipation by examining the accounts of participants who chose not to participate in screening offered by a national research trial of atrial fibrillation (AF) screening in England (SAFER: Screening for Atrial Fibrillation with ECG to Reduce stroke). AF is a heart arrhythmia that increases in prevalence with age and increases the risk of stroke. Systematic screening for AF is not a nationally adopted programme within the United Kingdom; it provides a unique opportunity to explore screening nonparticipation outside of the norms and values attached to existing population-based screening programmes. We interviewed people aged over 65 (n = 50) who declined an invitation from SAFER and analysed their accounts thematically. RESULTS: Beyond practical reasons for nonparticipation, interviewees challenged the utility of identifying and managing AF earlier. Many questioned the benefits of screening at their age. The trial's presentation of the screening as research made it feel voluntary-something they could legitimately decline. CONCLUSION: Nonparticipants were not resistant to engaging in health-promoting behaviours, uninformed about screening or unsupportive of its potential benefits. Instead, their consideration of the perceived necessity, legitimacy and utility of this screening shaped their decision not to take part. PATIENT OR PUBLIC CONTRIBUTION: The SAFER programme is guided by four patient and carer representatives. The representatives are embedded within the team (e.g., one is a co-applicant, another sits on the programme steering committee) and by participating in regular meetings advise on all aspects of the design, management and delivery of the programme, including engaging with interpreting and disseminating the findings. For the qualitative workstream, we established a supplementary patient and public involvement group with whom we regularly consult about research design questions. / The SAFER study is funded by the National Institute for Health and Care Research (NIHR) Programme Grants for Applied Research (grant reference number RP-PG-0217-20007) and School for Primary Care Research (SPCR-2014-10043, project 410). The views expressed are those of the authors and not necessarily those of the NIHR or the Department of Health and Social Care. SH, AP and JB are based in The Healthcare Improvement Studies Institute (THIS Institute), University of Cambridge. THIS Institute is supported by the Health Foundation, an independent charity committed to bringing about better health and healthcare for people in the UK. GT was funded as an Academic Clinical Fellow by Health Education England East of England and subsequently by NIHR School of Primary Care Research Bridging Funding (grant number S200) while undertaking work for this paper. The views expressed are those of the authors and not necessarily those of the Health Education England, the NHS, the NIHR or the Department of Health and Social Care. NA is supported by a Health Foundation Improvement Science Fellowship and by the National Institute for Health & Care Research (NIHR) Applied Research Collaboration East Midlands (ARC EM). The views expressed are those of the authors and not necessarily those of the NHS, the NIHR or the Department of Health and Social Care.