Right bundle-branch block (RBBB) in the patient with acute coronary syndrome is a marker of significant potential cardiovascular risk; the RBBB pattern in the patient with acute coronary syndrome identifies a subgroup of patients with quite high short- and long-term morbidity and mortality. Right bundle-branch block is not an uncommon finding on an electrocardiogram in the emergency department patient, noted incidentally and thus without clinical import or, conversely, encountered in the early phase of significant cardiovascular dysfunction. This review will address RBBB in the acute coronary syndrome setting.