We report six cases of segmental agenesis of the internal carotid a. discovered in adult patients: one case of cervical segmental agenesis (no. 1), one case of cervical and petrosal segmental agenesis (no. 2), two cases of vertical cavernous segmental agenesis (nos. 3 and 4) and two cases of distal segmental agenesis, one unilateral (no. 5) and the other bilateral (no. 6). The collateral pathways observed were: the ascending pharyngeal a. which constitutes an "intratympanic course" of the internal carotid a. (no. 1), an intercarotid anastomosis (no. 2), an arterial network at the base of the skull, the so-called "rete mirabile" (nos. 3 and 4) and the posterior communicating a. (nos. 5 and 6). Recognition of these rare dysgenesis relies upon the following radio-anatomic characteristics: reduced caliber of the a., inconsistent sparing of the carotid body, reduced diameter or even absence of the carotid canal and above all, the presence of collateral pathways. The collateral pathways allow an understanding of the segmental nature of carotid a. development and a distinction between congenital and acquired stenoses.