Supraclavicular brachial plexus block, due to its wide range of indications, is the most widely practiced procedure in anesthesiology. We experienced the case of a 45-year-old female patient who developed unilateral Horner's Syndrome after the use of supraclavicular brachial plexus block. The patient recovered spontaneously from the Horner's syndrome after 2 hours. If Horner's syndrome should occur, its etiology will need to be assessed. It is also important to assure the patient they will recover from the complication within a year.