We analyzed the high-fidelity left atrial and left ventricular pressures and the echocardiograms of the mitral valve, left atrium, and left ventricle in patients with complete heart block. During left ventricular diastole, the mitral valve opened, and no pressure gradient was observed between the left atrium and the left ventricle before the left atrial contraction. After the left atrial contraction, the mitral valve closed with persistent higher left ventricular than left atrial pressure. These findings indicated that during the left ventricular diastole in patients with complete heart block, the mitral valve closed after the left atrial contraction, which appeared to be maintained by the pressure gradient between the left ventricle and the left atrium.