The characteristic clinical features and morphological findings of five cases of (clinically not diagnosed) fusiform aneurysm of the basilar or vertebral arteries were recurring attacks of positional occipital headache, pain and stiffness in the neck, cranial nerve disturbances, expecially oculomotor palsies and anisokoria, nystagmus, attacks of nausea, vomiting and sweating, tachycardia, pyramidal tract symptoms, and pareses. Severe hypertension had been present in four instances. The aneurysm, which is usually thrombosed, pressed against the pons and medulla oblongata as a space-occupying mass. In addition to hypertension and atheromatosis, congenital defect in the arterial wall are probably significant causative factor. To mistake an aneurysm for a cervical syndrome may be fatal to the patient.