Background: Acute pediatric encephalitis is a debilitating neurological disorder that develops as a rapidly progressive encephalopathy (usually in less than 6 weeks) due to brain inflammation, autoimmune encephalitis (AE) is a heterogeneous group of recently identified disorders. Despite severe and even prolonged neurologic deficits, dramatic improvements may occur with proper immunotherapy in some patients with AE. Anti-neuronal antibodies have been discovered in cerebrospinal fluid (CSF) of patients, Aim and Objectives: The aim of the current study was to test the relation of early diagnosis with treatment of N-methyl D- Aspartate Receptor autoimmune encephalitis. Patients and Methods: This is Prospective observational cohort study that was carried out at pediatric department, Faculty of Medicine, Zagazig University, the study was conducted on 36 cases, Results: There was no statistical significant difference between the studied groups with and without autoimmune encephalitis regarding results of CSF analysis (larger percentage without autoimmune encephalitis had abnormal CSF analysis). There is a statistical significant difference between the studied groups with and without autoimmune encephalitis regarding prognosis. Conclusion: CSF NMDAR antibodies are highly sensitive biomarker in prediction of mortality and poor prognosis of encephalitis in children.