Two cases of stroke and one of subarachnoid haemorrhage are reported in women of child-bearing age, in which histological examination of the cervical and intracranial arteries revealed idiopathic medial necrosis. In the first case there was a dissecting aneurysm of a vertebral artery in the neck, caused by medial necrosis of this vessels. Two further dissecting aneurysms were found of the cervical arteries and partial rupture of the posterior cerebral artery, as well as focal medial degeneration of the aortic arch. In the second case medial necrosis of the internal carotid artery led to partial rupture and thrombotic occlusion of this vessel. In the third case a massive subarachnoid haemorrhage occurred after several years of severe migraine. Histological examination of an "aneurysm" of a vertebral artery revealed complete rupture and focal medial necrosis. The scanty literature on this subject is reviewed and the significance of idiopathic medial necrosis in stroke and unexplained subarachnoid haemorrhage is emphasized.