✓ The origin and mechanism of rupture, repair, and growth of intracranial saccular aneurysms are reported in an investigation of 45 aneurysms (23 unruptured and 22 ruptured) found in 34 brain specimens. Gaps in the media at the bifurcations of cerebral arteries are important for aneurysmal formation. The walls of aneurysms smaller than 3 mm in diameter are mainly composed of endothelial cells, and fibrous tissue. When aneurysms grow larger than 4 mm, the walls become collagenous and extremely thin portions develop in their domes, forming potential rupture points. Immediately after the rupture, bleeding is stopped by clot and a new fibrin layer is formed around the rupture point. It is proposed that the cerebrospinal fluid has a special accelerating action in clot formation. This fibrin layer is weak, and repeat rupture occurs within 3 weeks after the initial hemorrhage. However, after 3 weeks, rebleeding is rare due to the reinforcement of this layer, and capillaries appear in this new wall. Hemorrhages from these capillaries occur within and outside the new wall caused by the constant impingement of blood flow. In severe cases, the aneurysm ruptures again, but when the hemorrhages are slight, the aneurysm grows as the wall is thickened by repeated hemorrhages and their absorption. This may explain the growth mechanism of the aneurysm.