A 44-year-old man presented with painful Horner syndrome: severe periorbital pain, ptosis, and miosis of his right eye, with intact facial sweating. Lymphadenitis at the right side of his neck preceded the symptoms. MRI and magnetic resonance angiography showed thickening of the right internal carotid artery, extending from the bifurcation to the cavernous sinus, without evidence for dissection. The patient was treated with corticosteroids with immediate improvement. Control MRI scanning was normal after 6 weeks. We conclude that the painful Horner syndrome was caused by a reactive arteritis of the right internal carotid artery.