A 19-year-old man was admitted to our hospital for elective surgery to replace the ascending aorta. The patient's father had died suddenly of aortic dissection associated with Marfan syndrome. Because of the patient's family history and physical findings, we suspected Marfan syndrome and began regular observations of aortic size when he was an infant. When he was 18 years old, magnetic resonance cardiography showed a marked increase in the size of the ascending aorta, replacement of which was indicated. Computed tomography performed before the planned elective surgery incidentally revealed dissection of the ascending aorta, although the patient was asymptomatic. Thus, we failed to prevent the aortic dissection in this patient, although we had regularly observed the aorta since he was an infant. Surgery should have been performed earlier than it was. Our experience suggests more frequent observation and stricter indications for aortic surgery should be considered in Marfan syndrome.