Surgical specimens of 4 intracranial saccular aneurysms were studied by scanning electron microscopy. The internal surface of the aneurysms showed crater-like defects and cytoplasmic bridges. In some areas the endothelium was preserved, but its longitudinal convolutions were higher and thicker than those found in unchanged areas. On the damaged endothelial surface there was an increased number of blood cells. The adventitia resembled that of a normal cerebral artery. In conclusion the alterations observed are similar to those found in atherosclerosis and are most likely due to the high wear and tear provoked by the blood streaming into the aneurysm. The results of this study are in agreement with the findings of a companion transmission electron microscopy study and emphasize the importance of degenerative changes on the development and rupture of intracranial saccular aneurysms.