Fifty unfixed cerebral hemispheres were injected with polyester resin and dissected under the operating microscope to show the anatomy of the posterior communicating artery (PCoA). There was a single PCoA in every hemisphere. Eleven (22%) were of fetal origin, and 17 (34%) were hypoplastic. Infundibular dilatations were found in 5 (10%) of the arteries. The outer diameter (OD) of the PCoA at its origin was 1.5 +/- 0.8 mm on the right and 1.6 +/- 0.6 mm on the left. At the junction of the PCoA with the P1 segment of the posterior cerebral artery, the PCoA had an OD of 1.4 +/- 0.7 mm on the right side and 1.6 +/- 0.6 mm on the left. The total length of the PCoA was 12.7 +/- 3.2 mm on the right and 12.5 +/- 1.7 mm on the left side. PCoA branches originated from the superior (36%) or lateral (64%) surface of the PCoA and coursed superiorly, posteriorly, or laterally. These vessels supplied the paramedian perforated substance (21%), the tuber cinereum (16.8%), the sulcus between the optic tract and the tuber cinereum (14.4%), the circuminfundibular anastomosis (11.5%), the mamillary bodies (8.4%), the sulcus between the optic tract and the cerebral peduncles (7.6%), and the cerebral peduncles (5.7%). The largest and most constant branch of the PCoA was the premamillary artery. The number and size of the branches from the PCoA were independent of the size of the parent artery.