Introduction Efforts to explore the utility of neurofilament light (NfL) as a biomarker associated with disability progression in multiple sclerosis (MS) have accelerated in recent years in the absence of pharmacodynamic or treatment response markers for clinical trials or patient care.1 The International Progressive MS Alliance stated in 2020 that serum NfL (sNfL) measurements may serve as a useful biomarker associated with progressive MS, although further work is needed to define the relative contributions of inflammatory activity and neurodegeneration to longitudinal changes in disability and sNfL.2 Using data from a large clinical trial of patients with secondary progressive MS (a phase 3, randomized, double-blind, placebo-controlled trial exploring the effect of natalizumab on disease progression in participants with Secondary Progressive Multiple Sclerosis [ASCEND in SPMS]; NCT01416181), we investigated whether sNfL could be used as a dynamic biomarker associated with progressive MS disease course. That is, we investigated whether longitudinal changes in sNfL concentration were associated with disability progression measures in the absence of relapses and magnetic resonance imaging (MRI) evidence of inflammatory activity