There is no accepted hypothesis explaining the mechanism of growth and the subsequent rupture of intracranial aneurysms. Both congenital and acquired factors are believed to contribute to the formation and development of intracranial aneurysms. Apoptosis, commonly observed under a wide range of physiological conditions, occurs in the various pathological situations including some vascular diseases. We discuss the contribution of apoptosis to the formation and the rupture of human intracranial aneurysm. Five aneurysms without surgical treatment from four autopsy cases suffering from subarachnoid hemorrhage were studied by a specific in situ nick-end labeling method for DNA breaks. Conventional hematoxylin and eosin staining, and van Gieson staining for elastic fiber were also performed. Nonatherosclerotic regions in the parent arteries of the aneurysms or contralateral arteries were examined for controls in the same manner. Many apoptotic cells with nuclear DNA fragmentation were recognized in neck and dome of aneurysms, while few findings for DNA breaks were available in control arteries. Evidence for apoptosis was present in the spindle-shaped cells constituting the thin wall close to the rupture point within aneurysmal dome. These results strongly suggest that apoptosis plays an important role not only in the development of intracranial aneurysms but also in the aneurysmal rupture.