Horner's syndrome is the triad of miosis, ptosis, and anhidrosis, resulting from disruption of the sympathetic pathways. This article describes an uncommon case of Horner's syndrome in a 22-year-old man after blunt trauma to the neck and chest without carotid artery dissection. The patient was brought to the emergency service after motorcycle fall. Neurologic examination revealed a patient presenting the score 15 at Glasgow Coma Scale. The left eyelid was 1-2 mm lower than the right. Carotid Doppler and angiotomography were undertaken and revealed no abnormalities of the carotid artery. CT disclosed a mediastinal hematoma extending to the left apex, compressing the left sympathetic chain. The understanding of this clinical entity may help the surgeon to make a better differential diagnosis in trauma patients in whom prompt diagnosis is critical to establish the correct treatment.