Background Mitral annular calcification (MAC) associated with Marfan syndrome is rare in comparison with that frequently found in elderlies with valvular disease. Case Presentation A 17-year-old woman with Marfan syndrome underwent mitral valve replacement for severe mitral regurgitation. Preoperative examination showed mitral valve prolapse and a dense C -shaped MAC. We evaluated MAC and adjacent area, respectively, in postoperative 18 and 38 years, and neither progression nor expansion of calcification was observed. Conclusion MAC associated with Marfan syndrome is more likely to be caused by stresses due to valve prolapse than by connective tissue disorder.