The anatomy of the thalamoperforating arteries located in the interpeduncular fossa must be well understood by surgeons to enable safe surgical treatment of basilar and posterior cerebral artery aneurysms. Therefore, we studied 30 posterior cerebral arteries obtained from 15 fresh adult cadaver brains. By filling the vertebral and internal carotid arteries of the brains with coloured latex, we found thalamoperforating arteries in 97% of the brains studied. The average number of arteries was two (range 0-5). Thalamoperforating arteries were classified into four different types according to their origin at the P1 segment: type I (bilateral multiple), 20%; type II (unilateral multiple, unilateral single), 33%; type III (bilateral single), 40%; type IV (one side multiple, the other side with no branches), 7%. In conclusion, it is important to bear in mind that these arteries can be the unilateral single type, and that they may be absent on the other side. Unilateral single arteries are very significant for surgical technique.