HS was present in 191 (38.5%) of the patients with ICAD and 36 (13.4%) of the patients with VAD (p < 0.001). We analyzed 765 patients (n = 496 with ICAD, n = 269 with VAD, n = 303 prospective, n = 462 retrospective). Crude odds ratios (ORs) with 95% confidence intervals and ORs adjusted for age, sex, center, arterial occlusion, bilateral dissection, stroke severity, and type of antithrombotic treatment were calculated. Patients with HS (HS+) were compared with HS- patients. The presence of HS was systematically assessed using a standardized questionnaire. In this observational study, characteristics and outcome of patients with ICAD or VAD from the CADISP (Cervical Artery Dissection and Ischemic Stroke Patients) database were analyzed. To study the prognostic importance of Horner syndrome (HS) in patients with internal carotid artery dissection (ICAD) or vertebral artery dissection (VAD). Besta Neurological Institute, via Celoria 11, Milan, Italy University of Caen Basse Normandie (E.T.), INSERM U919, Department of Neurology, CHU Côte de Nacre, Caen, France Department of Neurology (J.J.M.), Sanatorio Allende, Cordoba, Argentina and Neurorehabilitation Unit (S.T.E.), Geriatric Competence Center, Felix Platter-Spital, Basel, Switzerland.