Immune aging occurs in the elderly and in autoimmune diseases. Recently, IgD− CD27−(double negative, DN) and CD21− CD11c+(CD21 low) B cells were described as age-associated B cells with proinflammatory characteristics. This study investigated the prevalence and functional characteristics of DN and CD21 low B cells in multiple sclerosis (MS) patients. Using flow cytometry, we demonstrated a higher proportion of MS patients younger than 60 y with peripheral expansions of DN (8/41) and CD21 low (9/41) B cells compared with age-matched healthy donors (1/33 and 2/33, respectively), which indicates an increase in age-associated B cells in MS patients. The majority of DN B cells had an IgG+ memory phenotype, whereas CD21 low B cells consisted of a mixed population of CD27− naive, CD27+ memory, IgG+, and IgM+ cells. DN B cells showed similar (MS patients) or increased (healthy donors) MHC-II expression as class-switched memory B cells and intermediate costimulatory molecule expression between naive and class-switched memory B cells, indicating their potential to induce (proinflammatory) T cell responses. Further, DN B cells produced proinflammatory and cytotoxic cytokines following ex vivo stimulation. Increased frequencies of DN and CD21 low B cells were found in the cerebrospinal fluid of MS patients compared with paired peripheral blood. In conclusion, a proportion of MS patients showed increased peripheral expansions of age-associated B cells. DN and CD21 low B cell frequencies were further increased in MS cerebrospinal fluid. These cells could contribute to inflammation by induction of T cell responses and the production of proinflammatory cytokines.