Arnold-Chiari malformations are a group of congenital or acquired defects associated with the displacement of cerebellar tonsils into the spinal canal. First described by Chiari (1891), this has various grades of severity and involves various parts of neuraxis, for example, cerebellum and its outputs, neuro-otological system, lower cranial nerves, spinal sensory and motor pathways. The symptomatology of Arnold-Chiari malformations may mimic multiple sclerosis, primary headache syndromes, spinal tumours and benign intracranial hypertension. We highlighted a case of Chiari type I malformation, who presented with posterolateral ataxia associated with significant vitamin B₁₂ deficiency. The patient was supplemented with vitamin B₁₂ injections and showed remarkable improvement at follow-up after 3 months.