Objective: To report a patient with agenesis of the right internal carotid artery associated with ipsilateral, congenital Homer's syndrome. Design: Case report. Methods: A 30-year-old woman, with a past history of migraine headaches, underwent neuro-ophthalmologic and neuroradiologic evaluation for transient visual obscurations and congenital Homer's syndrome. Results: A right, third-order neuron Homer's syndrome was confirmed with 1% hydroxyamphetamine topical drops. Cranial magnetic resonance imaging revealed an absent right internal carotid artery flow void, computed tomography demonstrated absence of the right carotid canal, and cerebral angiography confirmed absence of the right internal carotid artery. No atheromatous lesions were found and the results of coagulation studies were normal. Conclusions: Agenesis of the internal carotid artery is a rare cause of congenital Homer's syndrome. The cause of transient visual blurring in the current patient remains unproven. (C) 2000 by the American Academy of Ophthalmology.