Introduction: Only two previous reports of agenesis of the internal carotid artery with a coexistent Horner's syndrome are to be found in the medical literature. Case report: We report a case of a male child s een initially for what was presumed to be a traumatic superficial temporal artery aneurysm. MRA incidentally revealed ipsilateral agenesis of the internal carotid artery. This child's physical exam included ipsilateral Horner's syndrome. Conclusion: This case report illustrates that agenesis of the internal carotid artery does indeed cause Horner's syndrome in some cases. The clinician who diagnoses a patient with Horner's syndrome should remember that agenesis of the internal carotid artery may be the cause as this entity can be associated in itself with other pathologies of the central nervous system. © Springer-Verlag 2005.