BACKGROUND: Different types of ventricular septal defects (VSD) open to the inlet of the right ventricle. The atrioventricular conduction axis is markedly different within these subtypes, a feature of great surgical importance. To clarify these relationships, we have studied hearts with such VSDs from the Idriss archive at Ann and Robert H. Lurie Children's Hospital of Chicago. METHODS: We selected hearts from the archive showing the different variants of inlet VSD, photographing them to show the presumed disposition of the atrioventricular conduction axis as based on previous histology studies. We differentiated between perimembranous defects, muscular defects, perimembranous defects with straddling of the tricuspid valve, and atrioventricular septal defects with shunting confined at the ventricular level. RESULTS: The atrioventricular conduction axis is different in the four types of inlet VSDs. In perimembranous defects opening to the inlet of the right ventricle, the axis is positioned to the right hand of the surgeon operating through the tricuspid valve, whereas it is to the left hand with the muscular inlet defect. In patients with straddling tricuspid valve, the axis arises from an anomalous posteroinferior atrioventricular node, whereas in patients with atrioventricular septal defect with exclusive ventricular shunting, the axis arises at the crux of the heart from a node located in an inferiorly displaced nodal triangle. CONCLUSIONS: An appreciation of these relationships should help surgeons avoid the conduction system when closing inlet VSDs.