STUDY DESIGN A case report is presented. OBJECTIVES To illustrate a rare cause of atraumatic vertebral artery dissection resulting from anomalous entry of the vessel at the C3 transverse foramen induced by normal physiologic head and neck motion, and to review vertebral artery anatomy and mechanisms whereby it is vulnerable to pathologic compression. SUMMARY OF BACKGROUND DATA The vertebral artery usually enters the transverse foramen at C6. Rarely, the artery enters at C5 or C4. Only one prior case with entry at C3 has been reported. That patient experienced recurrent quadriplegia and locked-in syndrome caused by vertebral artery obstruction. A 27-year-old woman with a history of classic migraine experienced neurologic symptoms on three occasions related to physiologic neck and arm movements. Magnetic resonance angiogram was not diagnostic, but standard arteriography demonstrated anomalous vertebral artery entry into the C3 transverse foramen and focal dissection. METHODS Pertinent literature and the patient's history, physical examination, and radiologic studies were reviewed. RESULTS Standard cervico-cerebral arteriogram demonstrated focal dissection at C4 and thromboembolic complications in distal vertebral and basilar arteries. Initially, diagnosis by magnetic resonance angiogram was elusive. However, arteriography allowed prompt diagnosis followed by anticoagulation with resolution of neurologic symptoms. CONCLUSIONS Vertebral artery dissection without trauma is rare, but should be considered when neurologic symptoms accompany physiologic cervical movements. For cases in which vertebrobasilar thromboembolic ischemia is suspected, magnetic resonance angiogram may prove inadequate for demonstrating the causative vascular pathology. Therefore, standard cervico-cerebral arteriography should be performed.