Objectives: Given the increasing popularity of long-distance triathlon events amongst amateur athlete and the difficulty for emergency physician to address cardiovascular complaints in the context of exercise, this study aims to: -Identify the prevalence of electrocardiographic abnormalities before and after a long distance triathlon in a cohort of participants using the Seattle criteria. -Identify the acute changes that occur on their ECGs at the finish line of a long-distance triathlon. Methods: This prospective observational study examines the prevalence of selected standard 12-lead ECG findings, the Seattle criteria, in asymptomatic athletes before and after the completion of a long-distance triathlon. Results: Of 99 ECGs obtained prior to the race, 28 were abnormal, for a pre-race prevalence of 28.3% (95% CI (20.4, 37.8)). Of the 72 ECGs post-race, 12 were abnormal, for a post-race prevalence of 16.7% (95% CI (9.8, 26.9)). We did not observe any athletes with marked repolarization abnormalities. Common findings were increased QRS voltage significant for left ventricular hypertrophy (LVH) (24 (24.2%) pre-race, 10 (14.1%) post-race), early repolarization (21 (21.2%) pre-race, 19 (26.8%) post-race) and incomplete right bundle branch block (RBBB) (8 (8.1%) pre-race, 11 (15.5%) post-race). McNemar test showed no agreement between the ECG pre and post results (Chi-square =6.54, p = 0.01), suggesting a possible effect of the race on ECG findings. We observed a trend to normalization of athletes ECGs with acute exercise. Conclusion: Long-distance endurance exercise might acutely affect the ECGs findings in asymptomatic athletes and abnormal ECG findings were common in our cohort of athletes. Physicians providing care to long-distance athletes should interpret ECGs in this population prudently.