Although thyroid goiter is a common condition, it rarely results in Horner's syndrome. We report a case of a patient with an intrathoracic multinodular goiter complicated by Horner's syndrome. Benign thyroid disease was confirmed pathologically, and the patient's symptoms improved after surgery. In the literature, the major cause of Horner's syndrome is neoplasia, with malignant lesions being twice as frequent as benign tumors. An extensive review of the literature demonstrates a different repartition for thyroid neoplasia: including our case, 38 cases of Horner's syndrome secondary to a benign thyroid tumor are described, against only 8 cases caused by a thyroid carcinoma. We conclude that contrary to the commonly held opinion, Horner's syndrome is more often due to benign thyroid diseases than to thyroid malignancies.