Interatrial block is prolonged conduction between the right and left atria. A P-wave duration of > 120 ms identifies such blockage and is associated with wide, notched, or biphasic P waves. Jarath and Spodick1and Asad and Spodick2 studied two general hospital populations and reported a 47% prevalence of interatrial block in patients who were in sinus rhythm among patients of all ages, and 59% in patients ≥ 60 years of age. This is a much higher prevalence than the figures cited by many previous investigators. Presumably, therefore, this finding may have been widely overlooked, and indeed many textbooks in general medicine and cardiology do not mention interatrial block. Underreporting may have occurred by restricting studies to lead 2 alone or to only one or two other leads. This phenomenon can appear in any lead, and since sensitivity increases with the number of leads used, a full 12-lead ECG should be viewed.