A 5-year-old, previous healthy, girl developed a large hematoma of the right cerebral hemisphere and died. An aneurysm was strongly suggested. As previously found in adult aneurysm patients, histological and morphometric investigations showed an abnormality of the reticular fibers of the cerebral arteries. We suggest that arterial deficiency of reticular fibers is of importance in formation and rupture of saccular aneurysm, also in childhood.