NfL levels predominantly increase at disease onset in MOG-Abs-associated disorders

•NfL levels are increased in patients with MOGAD at onset•NfL levels decrease over time in most patients with MOGAD•NfL levels do not increase significantly during relapses in patients with MOGAD•Axonal damage predominantly occurs at onset in patients with MOGAD The unpredictable course and uncertain impact of relapses make treatment strategies of anti-myelin oligodendrocyte glycoprotein antibodies associated disorders (MOGAD) challenging. We analysed neurofilament light chain levels (NfL) in onset and follow-up sera of 18 patients with MOGAD to clarify the timing of axonal damage. In comparison with disease onset values (median 8.9 pg/mL, range 1.8-97), NfL levels remained stable or decreased in most follow-up measurements (n=52, median 6.7 pg/mL, range 0.2-207), including those measured on relapses. The predominant axonal damage occurs during onset, which could be the main driving factor of final disability, with subsequent relevant clinical and therapeutic implications.