A 60-year-old man was admitted at his local hospital for persistent chest pain and suspicion of aortic dissection. No evidence of aortic dissection or intimal disruption was noted by means of computed tomography and transesophageal echocardiography. A localized intramural hematoma of the ascending aorta was found. He was first treated medically and remained asymptomatic for 8 days when he developed a new episode of chest pain. He was found to have an acute type A dissection by computed tomography. He underwent graft replacement of the ascending aorta and had an uneventful post-operative course. This case report describes the development of true aortic dissection in a patient who previously had a localized intramural hematoma.