Cantó E, Barro C, Zhao C, et al. - In this cohort study of 607 individuals with multiple sclerosis (MS), experts assessed the potential of serum neurofilament light chain (sNFL) measurements as a biomarker of disease activity and progression in a longitudinal MS data set. The study discovered that statistically important relationships of sNFL with relevant clinical and neuroimaging outcomes in MS were verified and stretched, sustaining the potential of sNFL as an accurate substitute for ongoing MS disease activity. In this data set of individuals with MS who received early treatment, for relapse activity and long-term disability progression, the prognostic power of sNFL was insufficient.