The authors report a case of bacterial endocarditis from unidentified bacteria, responsible for a rupture of chordae of the mitral valve in a 4 month-old infant with a previously normal heart. Bidimensional echocardiography led to diagnosis. The importance of the mitral loss thus created made a mitral plasty necessary with cardiopulmonary by-pass at age 2 years. Ultrasonographic data were then confirmed. Postoperative result was excellent. The features of acute bacterial endocarditis in infancy are reviewed.