An operative complication in a patient with a true posterior communicating artery aneurysm: case report and review of the literature. Intraoperative oculomotor nerve injury in a patient with a true posterior communicating artery aneurysm is reported in detail. A comparison of internal carotid artery aneurysms at the posterior communicating artery junction with true posterior communicating artery aneurysms deserves special attention, because the vascular relationships of the aneurysm are more complex. A clip along the internal carotid artery does not occlude blood flow to the aneurysm, and the aneurysmal neck and the distal posterior communicating artery are closer to the oculomotor nerve. This is the 27th reported case of a true posterior communicating artery aneurysm. The incidence of true posterior communicating artery aneurysms ranges from 0.1 to 2.8% of all aneurysm patients. Such aneurysms constitute 4.6 and 11% of so-called posterior communicating aneurysms in two series. Difficulty associated with a preoperative diagnosis has been documented in at least 4 cases. An awareness of this rare aneurysm is stressed in order to avoid operative complications. 