We report the case of a large dissecting aneurysm of the anterior cerebral artery revealed by cerebral infarction in 38-year-old man. The volume and aspect of the aneurysm initially led us to the diagnosis of saccular aneurysm. Given the complete thrombosis, the risk of bleeding was low and antithrombotic therapy was started. Surgery could be discussed later. However radiological monitoring by MRI (magnetic resonance imaging) showed a rapid decrease in volume of the aneurysm. The final angiography found an aspect of stenosis followed by a little arterial dilatation. The diagnosis of dissecting anterior cerebral aneurysm was a posteriori established.