Horner's syndrome is the collection of clinical signs caused by sympathetic denervation of structures of the head. Lesions can be in the brainstem, spinal cord, thoracic outflow, sympathetic trunk, cranial cervical ganglion, or postganglionic axons in the head. The most common clinical signs in horses are sweating in the acute disease and prominent ptosis with decreased angle of the eyelashes. Associated clinical signs in associated structures allow the site of the lesion to be determined and a therapeutic and prognostic plan to be made. Polyneuritis equi is an uncommon neurologic disease of mature horses and ponies, principally characterized by clinical signs relating to chronic granulomatous inflammation of the cauda equina. Involvement of other nerves, particularly cranial nerves, is recognized, however, and the term polyneuritis equi is preferable and has superceded the older descriptive name of neuritis of the cauda equina.