OBJECTIVE: An intracranial aneurysm is an important acquired cerebrovascular disease that can cause a catastrophic subarachnoid hemorrhage. Atherosclerosis is one of possible mechanism, but its contribution to aneurysm formation is unclear. Human apolipoprotein E(apoE) is best known for its arterial protection from atherosclerosis. In this study we observe apoE expression in experimental cerebral aneurysms of rats to elucidate the role of apoE in the process of cerebral aneurysm formation. METHODS: Twenty-four male 7-week-old Sprague-Dawley strain rats received a cerebral aneurysm induction procedure. One month(12) and three months(12) after the operation, the rats were killed, their cerebral arteries were dissected, and the regions of the bifurcation of the right anterior cerebral artery-olfactory artery (ACA-OA) bifurcations were examined histologically and immunohistochemically. RESULTS: In the 1 month group (n=12), the ACA-OA bifurcation showed no aneurysmal change in 7 rats and early aneurysmal change in 5 rats. In the 3 months group (n=12), the bifurcation showed no aneurysmal change in 2 rats and an advanced aneurysm in 10 rats. ApoE expression were in 3 specimen in early aneurysmal change, but not in advanced aneurysms. CONCLUSION: ApoE expression in early aneurysmal wall suggests a possible role for apoE in early events leading to aneurysm formation. Further studies are necessary to elucidate the exact role of apoE in the pathophysiology of cerebral aneurysm.