OBJECTIVE: Niemann-Pick Type C (NPC) is a genetic neurodegenerative lysosomal storage disorder commonly associated with psychiatric symptoms and delays to accurate diagnosis and treatment. This study investigated biomarker levels and diagnostic utility of plasma neurofilament light chain (NfL) and glial fibrillary acidic protein (GFAP) in NPC compared to healthy controls. METHODS: Patients with NPC were recruited from a specialist assessment and management service. Data was available from an age and sex-matched healthy control group. NfL and GFAP were measured on Quanterix Simoa HD-X analysers and groups compared using generalised linear models. NfL levels were compared to, and percentiles derived from, recently developed NfL reference ranges. RESULTS: Plasma NfL was significantly elevated in 11 patients with NPC compared to 25 controls (mean 17.1pg/mL vs 7.4pg/mL, p<0.001), and reference ranges (all >98th percentile). NfL distinguished NPC from controls with high accuracy. GFAP levels were not elevated in NPC (66.6pg/mL vs 75.1pg/mL). DISCUSSION: The study adds important evidence on the potential diagnostic utility of plasma NfL in NPC, extends the literature of NfL as a diagnostic tool to differentiate neurodegenerative from primary psychiatric disorders, and adds support to the pathology in NPC primarily involving neuronal, particularly axonal, degeneration.