Acute aortic dissection with intramural hematoma has been believed to have a good prognosis, but we have encountered the transition of this entity to a classic dissection or aneurysm. We report the serial computed tomography (CT) features in 27 cases of acute aortic dissection with intramural hematoma. Eleven patients (40.7%) developed a classic dissection or aneurysm during follow-up. Four patients (14.8%) showed transition to a classic dissection without resolution of the intramural hematoma; each had a dilated ascending aorta measuring >5 cm in diameter on the initial CT. One case (3.7%) developed an enlarging aneurysm without resolution of the intramural hematoma. In 19 cases (70.4%), the hematoma resolved; among these 19, the aortic diameter was significantly larger (p< 0.01) than those in a normal control group. Two of these 19 later developed an aneurysm, and four developed a classic dissection. This entity often (40.7%; 11 of 27) required surgical intervention or periodic follow-up CT examinations, particularly with a dilated ascending aorta of >5 cm in diameter.