The authors report the case of a child with acute lymphoblastic leukaemia and hypereosinophilia complicated by left sided endomyocardial fibrosis. Despite the need for urgent treatment and severe mitral valve disease, a complex mitral valvuloplasty was performed, consisting of mobilisation and reconstruction of the posterior leaflet, burying the chordae with plicature of the papillary muscle of the anterior leaflet associated with an annuloplasty. Decortication was performed by a transvalvular approach. The technique of mobilisation-reconstruction of the posterior mitral leaflet with a pericardial patch should widen the indication of conservative surgery in this condition, in the presence of dominant mitral regurgitation in the child.