Infundibular dilatation is funnel-shaped symmetrical enlargement that occurs at the origin of cerebral arteries and which is apparent on 7 to 25% of normal angiograms. Infundibular dilatation is frequently considered a normal anatomic variation of no pathologic significance. The authors report a case in which an aneurysm developed on an infundibular dilatation of the posterior communicating artery [PComA]. A 72-year-old woman presented with severe headache, nausea, and vomiting. Digital subtraction angiography showed a saccular aneurysm arising from the origin of the left PComA. Operative findings revealed the aneurysm and infundibular widening of the right PComA. The aneurysm was successfully obliterated. Whether infundibular dilatation is a pre-aneurysmal state or a benign dilatation is controversial. However, we believe infundibular dilatation of the PComA in this case may have served as a pre-aneurysmal lesion.