The case of a patient with large pseudoaneurysm of the mitral-aortic intervalvular fibrosa following a blunt chest trauma is presented. A two dimensional echocardiographic study revealed a large aneurysmal sac situated between the posterior aortic root and the left atrium, which expanded in systole and partially collapsed in diastole. An echo-free space which represented the mouth of the aneurysm was seen just below the posterior aortic cusp in the junctional zone between the two valves, called mitral-aortic intervalvular fibrosa. Nuclear magnetic resonance imaging showed a better resolution of the echocardiographic feature. Cardiac catheterization and surgery confirmed the diagnosis.