BACKGROUND: Horner's Syndrome is defined by myosis, enophthalmos, lack of sweating and eyelid ptosis, as well as vascular dilatation of one half of the face, caused by damage of the ipsilateral cervical sympathetic chain. It is known that Horner's syndrome is an unusual complication of thyroidectomy and selective lateral neck dissection. Its exact pathophysiology is not totally explained and its treatment remains conservative.CASE PRESENTATION: A 27-year-old man developed one-sided partial eyelid ptosis, enophthalmos and myosis two hours after a total thyroid gland excision and a selective lateral neck dissection for papillary carcinoma. A clinical diagnosis of Horner's syndrome was formed. He was treated conservatively and presented with an incomplete recovery at a 2-month follow up.CONCLUSIONS: The present case report underlines the adjacent anatomical correlation between the thyroid gland, the celluloadipose tissue and the cervical sympathetic trunk throughout thyroidectomy and selective lateral neck dissection. Every surgeon should be familiar with the potential complications in order to preoperatively counsel patients, as well as avoid them during the surgical procedure.