Non-traumatic intracranial dissecting aneurysm (IDA) has been recently reported with increasing frequency and is recognized as a possible cause of subarachnoid hemorrhage. However, the pathogenesis of this disease is still unclear. Cystic medial necrosis (CMN) is known to be a cause of aortic dissection, especially in Marfan's syndrome. Presented herein is the case of a 49-year-old man who had IDA of the right anterior cerebral artery and abdominal aortic dissection without Marfan's syndrome. Histological examination showed medial degenerative changes with the accumulation of acid mucopolysaccharides in various intra- and extracranial arteries. Coexistence of dissecting aneurysms in the anterior cerebral artery and aorta suggests the presence of underlying pathogenesis that is common to these two dissection processes.