OBJECTIVES: Herpes simplex virus 1 encephalitis (HSE) is the most common infectious encephalitis in developed countries. We aimed to evaluate the association of serum neurofilament light chain (sNfL) with disease severity, outcome and secondary anti-neuronal autoantibodies in a retrospective cohort study. METHODS: We retrospectively identified 30 patients with HSE and 132 controls (bacterial meningoencephalitis BM n = 27, non-bacterial meningitis NBM n = 33, healthy controls = 72). sNfL concentration was tested in all available samples (N = 231) using single molecule array (SiMoA) technology. Screening for secondary anti-neuronal autoantibodies was performed using rat brain immunohistochemistry, cell-based assays for anti-neuronal surface antibodies and indirect immunofluorescence with non-permeabilized rat hippocampal neuronal cultures. RESULTS: Patients with HSE and BM had higher sNfL levels than NBM and healthy controls. Within the HSE group, higher onset sNfL levels were associated with need for mechanical ventilation and poorer outcome measured by modified Rankin Scale. Increased sNFL levels in follow-up samples (24 days after HSE onset, 95 % CI 15-33) were associated with development of secondary anti-neuronal autoantibodies. DISCUSSION: Our results suggest sNfL as a potential biomarker of neuroaxonal damage in HSE with the potential to identify patients at risk of developing secondary anti-neuronal autoantibodies, which might lead to secondary autoimmunity of central nervous system. Future studies are needed to evaluate diagnostic values and establish suitable cut-offs.