Although Horner syndrome is not a rare condition, little is known of the fact that the syndrome may, in some cases, alternate from one side to the other, which we will call "alternating Horner syndrome." Three patients with this peculiar syndrome had lower cervical lesions due to syringomyelia, Shy-Drager syndrome, and radiation myelopathy, respectively. A search of the literature revealed five instances of the same phenomenon. It was suggested that the alternating Horner syndrome can occur in patients with a lower cervical or an upper thoracic cord lesion without postganglionic or peripheral involvement.