BACKGROUND: The relevance of the atrial coronary anatomy in the pathogenesis of atrial arrhythmias and atrioventricular (AV) block complicating acute myocardial infarction (AMI) remains unclear. OBJECTIVES: We evaluated the location of the infarct-related coronary lesion relative to the principal atrial branches (ie, sinoatrial nodal, AV nodal, left atrial circumflex) in 454 patients with ST-elevation AMI in the CAPTORS II trial. METHODS: Patients underwent systematic 60-minute postfibrinolytic angiograms, and coronary anatomy was correlated with evidence of atrial arrhythmias and AV block on sequential electrocardiograms. RESULTS: Patients with either sinoatrial nodal or left atrial circumflex compromise (n = 34) had a higher incidence of "early" (ie, up to 90 minutes postfibrinolysis) atrial arrhythmias vs those without (23.5% vs 7.1%; P = .004). Patients with AV nodal compromise (n = 207) had a higher incidence of "early" AV block vs those without (12.1% vs 3.6%; P = .001). CONCLUSION: These findings support the etiological role of acute atrial ischemia in the development of early atrial arrhythmias and AV block complicating AMI.