BDNF and myeloid sarcoma: In this study, we compare BDNF levels in a large cohort of MS patients (pwMS) with age‐ and sex‐matched healthy controls (HC) and relate the levels of serum BDNF cross‐sectionally and longitudinally to comprehensive clinical, neuropsychological, and imaging measures to further explore the potential of serum BDNF as biomarker for pwMS.