Spontaneous dissection of the anterior cerebral artery is an unusual cause of subarachnoid hemorrhage. We present a case of a dissecting aneurysm of the anterior cerebral artery presenting with subarachnoid hemorrhage. A 51-year-old woman presented to our hospital with severe headache. Neurological examination demonstrated neck stiffness, decreased visual acuity of the left eye, and left ankle weakness. Computed tomographic scans showed subarachnoid hemorrhage. The initial cerebral angiogram demonstrated a slightly narrowed caliber and mild poststenotic dilation of the right A1 segment. A second cerebral angiogram 14 days later revealed no change in the focal narrowing of the proximal A1 segment but marked progression of the dilatation of the distal A1 segment. Right pterional craniotomy was performed. A sausage-like dilation of the right A1 segment was found with no definite mural hematoma. This abnormal right A1 segment was wrapped with a Sundt clip. A postoperative computed tomographic scan revealed Infarction of the right head of the caudate nucleus and the anterior limb of the right internal capsule. If a dissecting aneurysm is suspected, serial angiographic studies should be performed because of the possibility of dynamic changes over a short period.