The perforating branches of the internal carotid artery (ICA) were examined in 30 forebrain hemispheres. These branches were present in all the cases studied, and varied from 1 to 6 in number (mean, 3.1). Their diameters ranged from 70 to 470 microns (mean, 243 microns). The perforating branches arose from the choroidal segment of the ICA, that is, from its caudal surface (52.3%), caudolateral surface (34.1%), or caudomedial surface (13.6%). They rarely originated from the bifurcation point of the ICA (10%). The distance of the remaining 90% of the perforators from the summit of the ICA measured between 0.6 and 4.6 mm. The perforating branches most often originated as individual vessels, and less frequently from a common stem with another vessel or by sharing the same origin site with another perforator or with the anterior choroidal artery. The bifurcation of the ICA, which is a frequent site for cerebral aneurysms, is surrounded by many perforating branches. Hence, great care must be taken to avoid damage to these important vessels during operations in that region.