Objectives: We describe the structure of, and suggest an etiology for, the interatrial communication which can occur through the mouth of the coronary sinus. Based on the study of human embryos, we propose that the defect is best explained by dissolution of the wall of the coronary sinus adjacent to the left atrium, permitting shunting between the atriums through the right atrial orifice of the sinus. Background: An interatrial communication across the mouth of the coronary sinus defect was first described in 1965 by Raghib and colleagues, its existence being predicated on the basis of incomplete formation of the left “atriovenous fold”. Their hypothesis implies that the coronary sinus never develops, and thus the atrial septum itself is incomplete. Methods: We have studied the development of the coronary sinus in a series of human embryos. Based on this work, we present the anatomical findings in 6 specimens with varying degrees of dissolution of the walls of the coronary sinus, and ten specimens with isomerism of the right atrial appendages, in which the sinus has never been formed. Results: The coronary sinus defect is not a hole within the atrial septum, but a communication between the atriums through the mouth of the sinus. There was a range of defects in our series of specimens with usual atrial arrangement, extending from complete absence of the walls which normally separate the coronary sinus from the left atrium, to small fenestrations between this vessel and the left atrial cavity. In the hearts with isomerism of the right atrial appendages, however, we never observed an orifice of the coronary sinus. Thus, a coronary sinus defect cannot exist in this setting. Conclusions: Our findings indicate that the defect requires initial formation of the walls of the coronary sinus, but with subsequent dissolution of the wall adjacent to the left atrium. This produces a communication between the atriums through the mouth of the sinus.