Mitochondrial DNA (mtDNA) has been identified as a potential biomarker of multiple sclerosis (MS) disease activity and treatment response. In the new study, levels of mtDNA were measured in the cerebrospinal fluid of 48 people with MS before and after autologous haematopoietic stem cell transplantation (aHSCT; an aggressive immune reconstitution therapy for the disease) and 32 healthy individuals. Levels of mtDNA were higher among people with MS than among healthy control participants and correlated with several clinical measures of disease. After aHSCT, levels of mtDNA in the group with MS did not differ significantly from those in healthy control individuals.