The pathogenesis of intracranial aneurysms and spontaneous cervical artery dissection is incompletely understood but a primary arteriopathy, possibly similar in both disorders, may be of importance. To investigate the frequency of intracranial aneurysms in patients with spontaneous cervical artery dissection, the angiograms of 164 patients who were diagnosed at the Mayo Clinic as having spontaneous extracranial carotid or vertebral artery dissection were reviewed. Thirteen intracranial aneurysms were detected in nine (5.5%) of the 164 patients: eight (8.8%) of the 91 female patients and one (1.4%) of the 73 male patients. The frequency of intracranial aneurysms in these patients was significantly higher (p less than 0.01) than that observed in a recent angiographic study from the same institution, estimating the frequency of intracranial aneurysms in the general population (1.1%). The significance of these findings is discussed.