Internal carotid artery (ICA) hypoplasia is a rare congenital anomaly that is sometimes difficult to distinguish from acquired diseases associated with narrowing of the ICA. As the carotid canal develops in association with the ICA, a hypoplastic carotid artery should result in a hypoplastic carotid canal. We used skull base CT scans to measure the size of carotid canals in 4 patients with ICA hypoplasia (bilateral in 1) and in 30 consecutive controls. The diameter of the carotid canal was significantly smaller in the patients with carotid hypoplasia (2.4–3.0 mm, on the involved side) than in the controls (mean = 6.4 ± 0.8 mm, range = 5.2–9.8 mm) with no overlapping value (p < 0.001). The ratio of the largest to the smallest carotid diameter was greater than 2 in patients with unilateral hypoplasia, while it never exceeded 1.2 in controls. CT scan of the skull base is a simple and reliable tool for confirming the diagnosis of ICA hypopl