Circulating levels of IgM anti-CD64, an immunosuppressive antibody recently identified in long-term stable multiple sclerosis (MS) patients, were found to fluctuate over time in MS patients. Antibody-positive patients showed a significantly lower annualized relapse rate value as well as reached sustained disability worsening and had a relapse in a significantly longer median time than those without antibody. Disease-modifying therapies (DMTs) only were the covariate influencing both the relapse occurrence and the disability accrual. Serum IgM anti-CD64 levels are associated with maintenance of clinical stability in MS and may be tested as a candidate biomarker predictive of benign course and favourable long-term response to DMTs treatment.