Internal carotid artery (ICA) agenesis is rare and corresponds to the concomitant absence of ICA and the carotid canal. Two possible mechanisms involve the association between ICA agenesis and intracranial aneurysms: hemodynamic overload and hyperflow by demand of a given territory. We describe a case of Type I agenesis of the left ICA associated with fusiform aneurysm and fenestration of the basilar artery (BA).