A patient with relapsing multiple sclerosis (RMS) was treated with a standard immunomodulatory therapy, but due to ongoing disease activity was switched to rituximab. Relapses ceased, but secondary progressive MS (SPMS) eventually appeared, associated with new focal spinal cord white matter lesions. Cerebrospinal fluid (CSF) showed persistent oligoclonal bands (OCB) and clonally related B cells in CSF and peripheral blood. The treatment escalation approach failed to prevent evolution to SPMS, raising the question of whether initiation of B‐cell depleting therapy at the time of RMS diagnosis should be tested to more effectively address the immune pathology leading to SPMS.