We encountered a very rare lesion, which included a dissection between a prosthetic patch for repair of an incomplete atrio-ventricular septal defect (AVSD) and a pseudo-intima which had developed. This is the first report of a dissection between an atrial septum patch and pseudo-intima after mitral valve replacement (MVR) in a patient with infective endocarditis following an incomplete AVSD repair. Dissection of the left atrial wall or atrial septum after MVR is extremely rare although some case reports have been published.