Spontaneous dissection of the internal carotid artery as a cause of Horner's syndrome has only been recognized in recent years. The authors describe three patients with this condition. Associated symptoms included ipsilateral orbital and frontal headache (3 patients), neck and facial pain (2), amaurosis fugax (1), and dysgeusia (1). The symptoms resolved in all patients within three months, yet oculosympathetic paralysis has persisted. Diagnosis of carotid dissection required cerebral arteriography, and the angiographic features are presented. Patients were treated with platelet antiaggregants, and they have remained neurologically stable during follow-up (mean, 12 months; range, 10–14 months). © 1987, American Academy of Ophthalmology, Inc. All rights reserved.