Herpes zoster ophthalmicus is a disease in which the varicella-zoster virus replicates and produces inflammation in the skin of the face supplied by the sensory branches of the ophthalmic division of the trigeminal nerve. It can also cause a conjunctivitis, keratitis, uveitis, extraocular muscle paralysis, and acute retinal necrosis. We found only a single report of this disease as a cause of Horner syndrome. Here we report a case of herpes zoster ophthalmicus that progressed to a sixth nerve palsy and, subsequently, a Horner syndrome. We discuss how the anatomic relationship of the fifth, sixth, and sympathetic nerves in the cavernous sinus provides a route whereby the varicella-zoster virus may produce a Horner syndrome. To our knowledge this is the first fully documented case of Horner syndrome caused by herpes zoster ophthalmicus.