The success of mitral valve repair with annuloplasty depends on the repair technique and on the choice of a ring with appropriate dimensions related to mitral annulus. Intraoperatively, the choice of the prosthetic ring size is made by the surgeon using special gauges that are placed on the anterior mitral leaflet (having the trigons and commissures as anatomical findings), a method considered the gold standard for defining annulus size. However, it is performed in cardioplegia, without considering annulus shape and motion during cardiac cycle. Three-dimensional techniques are fundamental in evaluating mitral valve structure and its changes during cardiac cycle; three-dimensional transoesophageal echocardiography (3D-TOE) has been proven to be essential for the anatomo-functional characterization of mitral valve apparatus in patients undergoing surgically mitral valve repair. It also allows the measurement of quantitative parameters useful in determining the size of the annuloplasty ring, such as A2 scallop height, intertrigonal distance, intercommissural diameter and total annular perimeter size, that will support the surgeon's choice on the dimensions of the ring to be implanted. Cardiac computed tomography (CCT) plays a key role for device sizing in patients undergoing transcatheter mitral valve replacement; it provides isotropic sub-millimetre spatial resolution and it is the gold standard for geometric characterisation of the mitral valve and for assessment of the spatial relationship of mitral valve apparatus to adjacent anatomical structures. In patients undergoing mitral valve surgery it is foremost performed to exclude coronary artery disease, as stated by European Society of Cardiology (ESC) guidelines. Several studies have compared CCT with 3D-TOE in sizing the mitral valve apparatus: Shanks et al. have demonstrated a good correlation between the two techniques in measuring intercommissural diameter; Hirasawa et al. have demonstrated a good correlation in evaluating intertrigonal distance. Only one study has compared the measures of the annulus obtained by CCT with the prosthetic ring (Alkhadi et al.) and only for 9 patients the intercommissural diameter was compared to the prosthetic ring, obtaining a good correlation.