The left atrium is of considerable importance in the natural and surgical story of mitral valve stenosis. It is the first chamber to be affected when the ventricle is being filled; inside it, blood stasis and the pro-coagulant effect that stenosis causes can promote the formation of thrombi that are a considerable factor in the patient’s morbidity and mortality, regardless of the severity of stenosis. Atrial dilatation is the morpho-pathological substrate that triggers atrial fibrillation, which itself is an independent factor of morbidity and mortality. For these reasons the left atrium deserves its own chapter.