Ventricular septal dissection may rarely result from infective endocarditis, myocardial infarction or sinus of Valsalva aneurysm progression. A rare case that developed in a 66-year-old female after mitral valve replacement for severe mitral regurgitation with cordal rupture is presented and discussed. It resulted from a coronary artery fistula, from a septal branch, to a 6 cm wide saccular cystic cavity within the interventricular septum, which was detected by transthoracic echocardiography some months after surgery. Coronary arteries were anatomically normal. Coronary angiography was helpful to clarify the origin of the fistula. Later on the patient developed heart failure and atrial fibrillation, but she refused any further intervention. She has been followed up for more than 7 years and is presently stable on medical therapy. A review of the peculiar aspects of the case is done and the most relevant aspects and images are presented and discussed.