A 33-year-old man died following sudden unexpected rupture of a saccular thoracic aortic aneurysm. The patient had been treated as suspicious Behçet's disease (BD) for 9 years, however, the medication was discontinued at the age of 24. Autopsy disclosed multiple aneurysms at the descending thoracic and abdominal aorta. Histopathologically, infiltration of inflammatory cells and destruction with loss of elastic and muscle fibers of the aortic medial wall were seen. Taking the clinical course into account, all the postmortem findings were compatible with those of vasculo-BD. It is important that medical follow-up should be implemented even for childhood-onset patients who do not satisfy criteria of BD.