Agenesis, aplasia and hypoplasia of the internal carotid artery are rare congenital malformations. They are usually asymptomatic and incidentally discovered through ultrasound or imagistic tests. The aim of this study is to improve their management in our Departments. We report here the case of a 39-year-old woman addressed to our ambulatory in 2013 for benign symptoms like dizziness and headache. Imagistic findings (magnetic resonance imaging of the brain, and cervical spine, and magnetic resonance angiography of the head and neck) indicated a very rare condition: left internal carotid artery agenesis accompanied by the absence of the pre-communicant part of the left anterior cerebral artery and of the right posterior communicating artery. Internal carotid artery agenesis is an uncommon congenital anomaly and it could be misdiagnosed as stenosis/occlusion of this artery. This condition is important to be recognized due to the associated hemodynamic changes and in order to discover and evaluate other accompanying vascular malformations (aneurysms, collateral channels) and their life threatening potential risks (subarachnoid hemorrhage or ischemia). Also, it has a special importance in case of planning carotid or trans-sphenoidal hypophyseal surgery.