Two cases of severe mitral regurgitation and pulmonary hypertension immediately after birth are presented. Initial echocardiographic findings showed a thick atrial septum rather than a normal flap septum. The patients were tentatively diagnosed with premature restriction of the foramen ovale and congenital mitral regurgitation. Subsequently, the mitral regurgitation resolved over time with improvement of pulmonary hypertension. The postnatal ductus arteriosus closure process was considered to be the cause of the mitral regurgitation and pulmonary hypertension.