Spontaneous nontraumatic rupture of the ascending aorta occurred in a hypertensive patient. The clinical findings suggested acute aortic dissection, and echocardiography showed a large pericardial effusion. Computed tomography scanning did not indicate aortic dissection, but aortography in 3 projections revealed an area of intimal disruption similar to the niche of an ulcer. The patient underwent replacement of the ascending aorta and proximal aortic arch, and the postoperative course was uneventful.(Circ J 2003; 67: 461–463)