In the pivotal trials of natalizumab in the treatment of relapsing-remitting multiple sclerosis (AFFIRM and SENTINEL), a dramatic reduction in relapse rate, new or enlarging T2-hyperintense lesions, and mean number of gadolinium-enhancing lesions was observed. While both relapses and new MRI lesions were observed in these trials, there has been no comment on the presence of aggressive disease in the face of natalizumab treatment. I report a 31-year-old woman with relapsing remitting MS of 12 years duration who developed aggressive demyelinating disease four months after the initiation of natalizumab. The clinical worsening was accompanied by a significant increase in new large T2-hyperintense signal abnormalities and in both solid and C-shaped contrast-enhancing lesions. Neither the clinical severity nor the striking MRI abnormalities had been noted earlier in her disease course. Neutralizing antibodies to natalizumab were not detected. She subsequently responded to combination therapy of pulsed methylprednisolone and daily glatiramer acetate.