Purpose: To report a case of Horner's syndrome and dissection of the internal carotid artery after chiropractic manipulation of the neck. Methods: Case report. A 44-year-old woman with no prior ocular or vascular history presented with severe right-sided head and neck pain, ptosis, and miosis following chiropractic treatment for a strained right shoulder muscle. Results: Magnetic resonance angiography of the neck and brain revealed a dissection of the right internal carotid artery as well as a suggestion of subtle dissection in the right vertebral artery. No significant brain abnormalities were noted on magnetic resonance imaging. Pharmacological testing was consistent with preganglionic oculosympathetic damage. Conclusion: Acute, painful Horner's syndrome as a manifestation of vascular dissection may be associated with chiropractic manipulation of the neck.