We report a case of a true posterior communicating artery aneurysm. A 51-year-old male suffered a subarachnoid hemorrhage with severe headache and vomiting. A true posterior communicating artery aneurysm was recognized after repeated angiography on the seventh day. Right frontotemporal craniotomy was performed and the aneurysm was successfully clipped. The incidence of true posterior communicating artery aneurysms ranges from 0.1-2.8%, and 21 cases including our case have been reported in detail. There are no reported cases in which the aneurysm arises from the branching site of perforating arteries. In almost all cases the dome of the aneurysm projects inferiorly or posteriorly or laterally, so perforating arteries from the posterior communicating artery rarely interfere with dissection of the aneurysm or neck clipping. In a few cases, true posterior communicating artery aneurysms had been diagnosed as IC-PC aneurysms preoperatively, leading to intraoperative aneurysmal rupture or postoperative neurological deficit or death. In the cases of a fusiform aneurysm or an aneurysm of wide-based neck, there may be no other choice than trapping of the aneurysm. It is difficult to predict whether trapping causes postoperative ischemic complications.