Initial report retrieved on 27 September 2011 via a search of the scientific literature. This article reports a case of Brown-Sequard syndrome that occurred in a patient following the administration of trivalent influenza vaccine. The patient responded well to intravenous steroids and physical therapy. This is the first reported case in the literature. A 57-year-old female with a history of thoracic (T6 level) neuritis, right leg paresthesias, and progressive left leg weakness was admitted to the hospital approximately 10 days after receiving trivalent influenza vaccination. In addition, she had developed urinary retention 24 hours prior to admission. Her medical history was significant only for a febrile reaction to a previous influenza vaccination. There was no history of drug or egg allergies or recent illnesses. Medications on admission included estrogen and medroxyprogesterone acetate. Physical examination revealed normal vital signs, and the patient was oriented to time, place, and person. Cranial nerves were intact. The upper limb strength, sensation, and reflexes were normal bilaterally. The left lower limb had one-fifth strength, decreased reflexes, normal sensation, and upgoing toes. The right limb had normal reflexes and strength with decreased sensation to pin prick and light touch extending to T8-T9. Vibration was diminished below the knees bilaterally with normal position sensation. Cerebellar testing was grossly normal considering the weakness of the left leg. Gait was not assessed accurately due to her inability to stand independently. The bladder also was noted to be distended; the rest of the physical examination was essentially normal. Laboratory data were unremarkable except for the cerebrospinal fluid (CSF), in which the total protein was 34 mg/dL, glucose was 78 mg/dL, 9 red blood cells/pLL, 15 white blood cells/uL with 91% lymphocytes, 0% polyps, and 9% monocytes. the CSF protein electrophoresis was normal, and the CSF culture was negative. Myelin basic protein was 7.8 ng/mL (indeterminant). the rheumatoid factor, antinuclear antibodies, and erythrocyte sedimentation rate were normal, and the monospot and Lyme antibody titer was negative. Magnetic resonance imaging (MRI) of the brain revealed nonspecific white matter changes in the frontal horns only. Magnetic resonance imaging of the cervical and thoracic spine revealed abnormal signals with mild generalized edema in the spinal cord from C3-T5. The patient was treated with intravenous steroids for approximately 8 days, followed by an oral steroid taper for 10 days, and underwent physical and occupational therapy. On follow-up, she was found to have improved strength in the left limb with persistent and sensory deficits of the right flank and leg. Excellent abductors and adductors of the thigh were noted, as well as improved strength in the hip flexors. Hyperesthesia persisted at the left T9 level. Repeat MRI of the spinal cord revealed resolution of the edema with improved patchy hyperintense linear lesions slightly to the left of the midline from T1-T4, which correlates with the clinical scenario of Brown-Sequard left thoracic syndrome. Note: This case also described in a table in another literature report which is captured in case 2011-09459. This table contained other reported cases of post-influenza vaccination encephalomyelitis/ADEM published since 1982. The cases in this table which also contained a possible sanofi pasteur influenza vaccine are found in the following: E2005-00990, 2010-00355, 2011-09595, and 2011-09627.