The mechanics of rupture of mitral valve chordae have been difficult to elucidate because most surgical repair procedures and pathological examinations are performed after the rupture. In an excised anterior leaflet segment from a fibrotic mitral valve, chordae were observed in an initial phase of rupture. Microscopic sections showed that thinned, nearly ruptured chordal segments were actually chordal cores, containing highly aligned collagen fibers. The outer sheath of elastic fibers, loosely organized and circumferentially oriented collagen fibers, and endothelial cells that normally surrounds the collagen core apparently had retracted to the extreme ends of the thinned segment, resulting in a bulbous shape, as noted in the chordal rupture literature. These new observations lead us to propose that failure of the chordal outer sheath represents the first phase in a slow, two-part process leading to eventual chordal rupture.