OBJECTIVEAn intracranial aneurysm is an important acquired cerebrovascular disease that can cause a catastrophic subarachnoid hemorrhage. Despite modern therapy, most patients die or are left disabled as a direct result of a severe initial hemorrhage. The development of more effective treatment strategies depends on understanding the fundamental biology of cerebral aneurysms. The purpose of the present study is to determine whether inflammation or immunological reaction occurs in cerebral aneurysms.METHODSAneurysm tissue was collected at the time of microsurgical repair from 23 unruptured and 2 ruptured aneurysms (25 patients) and compared with 11 control basilar arteries harvested at autopsy. Immunohistochemistry was used to localize complement (C3c, C9), immunoglobulins (IgG, IgM), vascular cell adhesion molecule-1, macrophages and monocytes (CD68), T lymphocytes (CD3), and B lymphocytes (CD20).RESULTSComplement (C3c,P< 0.0001; C9,P= 0.0017), immunoglobulin (IgG,P= 0.0013; IgM,P= 0.031), vascular cell adhesion molecule-1 (P= 0.0022), macrophages (CD68,P= 0.004), and T lymphocytes (CD3,P= 0.0004) were all frequently present in the wall of aneurysm tissue but were rarely identified in control basilar arteries. A few B lymphocytes (CD20,P= 0.41) were found in aneurysm tissue, but none were found in the basilar arteries.CONCLUSIONExtensive inflammatory and immunological reactions are common in unruptured intracranial aneurysms and may be related to aneurysm formation and rupture.