BACKGROUND Aerobic exercise is uniformly accepted as one of the most important modifiable factors to improve cardiovascular health, but endurance athletic training poses a significant risk factor for development of atrial fibrillation (AFib) in middle-aged and older adults. Ubiquitous risk assessment tools (CHADS2 and CHA2DS2-VASc) and US Preventive Services Task Force guidelines do not presently account for this association. A case is presented which illustrates a dire outcome of undiagnosed AFib in an elderly male patient who had run many marathons. CASE REPORT An 80-year-old male with well-controlled hypertension and hypercholesterolemia and a history significant for running many marathons throughout his life was brought in via ambulance after being found down with head trauma by his wife at home. A short run of AFib was recorded on telemetry and electrocardiogram (ECG) and a review of previous ECGs revealed evidence of interatrial block (Bayes Syndrome), though the patient had no history of AFib or anticoagulation. This coupled with imaging indicated thromboembolic stroke to the left middle cerebral artery leading to right-sided hemiplegia and a subsequent fall to the right, causing right-sided head trauma and intracranial hemorrhage. His clinical course did not improve, and on his fifth day of admission he was transferred to comfort care, extubated, and succumbed to his injuries. CONCLUSIONS This case and the accompanying summary of evidence strongly encourage further investigation and a higher index of suspicion for AFib in asymptomatic older adults with a history of endurance athletic training.