We describe a 10 year old boy who presented with acute onset rapidly progressing encephalopathy. MRI revealed bilateral insular cortex and basifrontal involvement suggesting Herpes encephalitis.He was treated with acyclovir and his symptoms improved. Six months after the first hospitalization he reported back with two episodes of partial seizures. He was started on Valproate. A week after starting valproate he was readmitted with hyperammonemic encephalpathy, on further investigations Citrullinemia Type 1 was diagnosed. This case highlights a metabolic disorder which radiologically mimics herpes encephalitis. The management of the disorder differs remarkably from herpes encephalitis and hence its recognition and suspicion based on radiology is critical.