In the posterior cerebral artery (PCA) where ruptured aneurysms formed most frequently, the mRNA level of interleukin (IL)-1β was higher and the imbalance of matrix metalloproteinase (MMP)-9 and the level of the tissue inhibitor of metalloproteinase-2 (TIMP-2) were greater than at the bifurcation of the anterior cerebral artery-olfactory artery (ACA-OA) where no ruptured aneurysms were observed. This evidence concerns the gene MMP9 and aneurysm.