Primary progressive multiple sclerosis (PPMS), relapsing remitting MS (RRMS) and acute disseminated encephalomyelitis (ADEM) are clinically and immunopathogenetically distinct phenotypes of inflammatory demyelinating disorders of the central nervous system. Progression following RRMS is well described as secondary progressive MS. We report a patient with unexpected transition from long established PPMS to clinically and radiologically active RRMS after an ADEM-like fulminant demyelinating episode despite an immunosuppressive treatment preceding relapses. We note clearly accelerated brain atrophy after the RRMS course ensues. The unique disease course in this patient illustrates the dissociation of the biology and disability impact of relapses and progression.