This study of the posterior communicating artery (PCoA) in 126 cranial cavities of adult cadavers revealed some hitherto unreported anomalies. These included: the origin of one PCoA from the ophthalmic artery inside the optic canal; the presence of three PCoA's on one side; the occurrence of two posterior cerebral arteries of different origin on one side; and the existence of junctional dilatations at both ends of a PCoA. Junctional dilatation at the commencement of the artery was seen in 6.3% of cases. More common anomalies were a fetal type of posterior cerebral artery in 31.7% and macroaneurysmal dilatation of the PCoA in 39.7% of cases. The presence of multiple arteries makes possible normal circulation in cases involving obstruction to any one of them. The junctional dilatation or dilatation of any other part of the PCoA, if pre-aneurysmal, may leak or rupture to produce subarachnoid hemorrhage. Arteries pressing on the oculomotor nerve may produce diplopia, and those compressing the optic chiasm and tracts may cause visual defects.