A fluid dynamics model of mitral valve motion during diastolic filling of the left heart is described. Given a pulsed Doppler velocity pattern in the mitral annulus, the radius of circular mitral orifice, the length of leaflets and the end-systolic left ventricular volume, the numerical model predicts the time course of the mitral leaflets during diastole: the mitral valve M-mode echocardiogram. Results obtained by computer simulation have been validated with in vivo data. It is shown that mitral valve flow is essentially a fluid dynamics process of floating mitral valve leaflets with blood flow due to the atrioventricular pressure gradient. In addition, a partial opening of the mitral valve as the initial boundary condition is required to simulate the overshooting of the leaflets during early peak filling. Some back flow is a condition for perfect closing of the native mitral valve. The higher the unsteady character of mitral flow, the less efficient is the opening and closing processes of the mitral valve.