Purpose Congenital anomaly of the internal carotid artery (ICA) is a rare entity. It is usually discovered incidentally by color doppler carotid sonography, angiography, computerized tomography (CT), or magnetic resonance imaging of the head and neck region taken for some other reasons. The aim of this study was to detect congenital ICA anomalies, to delineate existing collateral vessels and to find out its incidence. Methods 1847 patients' CT angiography images of the head and neck region taken between May 2013 and February 2018 were retrospectively evaluated for ICA anomalies. Results We detected three cases (0.16%) with unilateral agenesis of ICA, bilateral agenesis of ICA and bilateral hypoplasia of ICA, respectively. Most patients are asymptomatic because of collateral cerebral circulation supplied by the communicating arteries of the circle of Willis, intercavernous anastomosis, communicating arteries from the external carotid artery, and by persistent embryologic arteries to the carotid artery territory. Conclusion Recognition of ICA anomalies has important implications during planned carotid or transsphenoidal surgery, in thromboembolic disease, and in the follow-up and detection of associated cerebral aneurysms.