In order to assess the value of NfL as a diagnostic and monitoring biomarker in SMA and evaluate whether elevated NfL levels are associated with SMA subtype or age, we quantified NfL concentrations in serum and CSF of children and adolescents with SMA carrying either 2 SMN2 copies or >2 SMN2 copies before and during therapy with nusinersen over a period of up to 34 months. Here, SMN2 is linked to proximal spinal muscular atrophy.