Jorge Correale, MD, Raúl Carrea Institute for Neurological Research (FLENI), Buenos Aires, Argentina, discusses the role of vitamin D in multiple sclerosis (MS). It is known that vitamin D levels can module MS risk, and some evidence suggests it can also modulate disease severity. Dr Correale talks about the impact of vitamin D on several immune mechanisms in MS, as well as its impact on treatment efficacy and differential benefits in men and women. This interview took place during the ACTRIMS Forum 2021.