Interatrial blocks, characterized by P-wave duration ≥120 ms, are a well described but poorly recognized cardiac rhythm disorder. They are caused by a conduction delay between the right and left atria and occur in pandemic proportions in unselected patients. Interatrial blocks correlate with atrial dysfunction and are a predictor of significant atrial arrhythmias, particularly atrial fibrillation, as well as embolic stroke, all-cause and cardiovascular mortality. Special attention to this cardiac rhythm disorder is required because of its pathologic implications.