The authors report a case of spontaneous internal carotid artery dissection with a fibromuscular dysplasia of the heterolateral artery. Etiopathogeny, anatomo-pathology, different arteriographic forms and treatment are discussed. The interest of this case is to be found in two aspects: The dysplasia seems to be at the origin of the dissection. It shows that the treatment hereby applied (extra-intracranial bypass) will be of good interest in such situation. The patient became and remain totally asymptomatic 45 months after the second operation.