Enlargement of left atrium occurs in patients with longstanding mitral valve disease due to chronic pressure and volume overload and occasionally left atrium reaches a massive enlargement, condition known as giant left atrium. It is most commonly associated with rheumatic mitral valve disease, both stenosis and regurgitation. This unique case deals with a 70-year-old woman who developed a giant left atrium due to a severe mitral regurgitation from complete prolapse of both mitral leaflets, as a consequence of previous undersized mitral ring annuloplasty.