We report a case of Horner's syndrome (HS) occurring as a complication after total thyroidectomy. Horner's syndrome is characterized by myosis, eyelid ptosis, enophthalmos, and lack of sweating, with vascular dilatation of the lateral part of the face, caused by damage of the cervical sympathetic chain. We found only 28 other reports of HS developing after thyroidectomy, and only seven of these patients recovered completely. Of the 495 thyroidectomies performed at our hospital between 1997 and 2007, only one (0.2%) was complicated by the development of HS. The patient was a 35-year-old woman who underwent total thyroidectomy for Basedow-Graves' disease. Horner's syndrome manifested on postoperative day 2, but without anhydrosis or vascular dilatation of the face, and the symptoms resolved spontaneously 3 days later. The possible causes of HS after thyroidectomy include postoperative hematoma, ischemia-induced neural damage, and stretching of the cervical sympathetic chain by the retractor. The prompt and complete recovery of this patient suggests that the cervical sympathetic chain was damaged by retractor stretching.