A rare case of a 14-year-old child with congenital mitral insufficiency secondary to hypoplasia of the posterior leaflet is reported. Echocardiography revealed the almost complete absence of the posterior mitral leaflet, which determined massive regurgitation. At surgical inspection the posterior leaflet was almost completely absent, represented only by tags of fibrous tissue that strictly adhered to the posterior annulus with a total absence of chordae inserting into the hypoplastic leaflet. The mitral valve was successfully repaired by restrictive annuloplasty, which gained a satisfactory surface of coaptation between the anterior leaflet and the primordial posterior structure, resulting in stable valve continence.