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To assess the impact of sampling bias due to reported as well as unreported exclusion of the target population in a multi-center randomized controlled trial (RCT) of ST-elevation myocardial infarction (STEMI). We compared clinical characteristics and mortality between participants in the DANAMI-3 trial to contemporary non-participants with STEMI using unselected registries. A total of 179 DANAMI-3 participants (8%) and 617 contemporary non-participants (22%) had died (Log-Rank: P < 0.001) after a median follow-up of 1333 days (range: 1–2021 days). In an unadjusted Cox regression model all groups of non-participants had a higher hazard ratio to predict mortality compared to participants: eligible excluded (n = 144) (hazard ratio: 3.41 (95% CI: (2.69–4.32)), ineligible excluded (n = 472) (hazard ratio: 3.42 (95% CI: (2.44–4.80), eligible non-screened (n = 154) (hazard ratio: 3.37 (95% CI: (2.36–4.82)), ineligible non-screened (n = 154) (hazard ratio: 6.48 (95% CI: (4.77–8.80). Sampling bias had occurred due to both reported and unreported exclusion of eligible patients and the difference in mortality between participants and non-participants could not be explained only by the trial exclusion criteria. Thus, screening logs may not be suited to address the risks of sampling bias. • Participants in the DANAMI-3 trial were not representative to the background population with STEMI. • Participants in the DANAMI-3 trial were not representative to the per protocol target population. • Sampling bias and unreported screening had occurred. • Screening logs may not be suitable for addressing issues of external validity. [ABSTRACT FROM AUTHOR]