Brown-Sequard syndrome (BSS) is a unilateral cord injury characterised by an ipsilateral motor deficit with contralateral pain and temperature hypoaesthesia. Although there are a variety of causes, the majority of cases are generally of neoplastic origin or are traumatic in origin. We describe a rare cause of Brown-Sequard syndrome as a result of post-traumatic arachnoiditis. Magnetic resonance imaging with the use of thin-slice high-resolution constructive interference in steady state (CISS) and T2-weighted spin-echo sequence were used to demonstrate the cause and appearance of the lesion in the spinal canal and was useful in the assessment and management of the patient. This case illustrates the usefulness of the CISS sequence in MRI for elucidating arachnoiditis.