Thirty-three injected human brains were examined, and anastomotic vessels were found in 26 (79%) of them. Anastomoses varied in number from one to six. The mean diameter was 148 microns, and the mean length was 3.3 mm. Anastomoses were observed among the following: the branches of the single thalamoperforating vessel of the posterior cerebral artery (PCA), the individual thalamoperforating arteries of the same PCA, the thalamoperforating vessels and branches of the basilar and superior cerebellar arteries on the same side, the peduncular branches of the PCA and ipsilateral branches of the basilar and superior cerebellar arteries, and the thalamoperforating arteries on one side and various contralateral vessels. Anastomoses could be important components of collateral circulation in occlusive cerebrovascular diseases.