An uncommon cause of cerebral ischemia in the territory of the posterior cerebral artery (PCA) is the combination of a fetal origin of the PCA and atherosclerotic disease in the internal carotid artery. This study compared the accuracy of CTA with DSA in the assessment of a fetal origin of the PCA. Patients in whom an intracranial DSA and CTA had been performed were reviewed. A fetal origin was defined as a normal-sized patent posterior communicating artery (PCoA) with hypoplasia or aplasia of the ipsilateral P1 segment. One hundred PCAs in 51 patients were analyzed. A fetal origin was present in ten vessels (10%, eight patients). CTA revealed all of them. CTA considered an additional three vessels as having a fetal origin, while DSA revealed a PCoA with the same diameter as the P1 segment of the PCA. Sensitivity and specificity of CTA in the assessment of a fetal origin could be estimated at 100 and 97%, respectively. Positive and negative predictive values were 77 and 100%, respectively. CTA can be considered a valid diagnostic tool for the assessment of a fetal origin of the PCA in patients with a cerebral ischemic event in the territory of the PCA.