The objective of this case report is to document the possibility that immunoablative doses of cyclophosphamide may provide a long-term remission of multiple sclerosis (MS). We report the case of a 48-year-old woman with definite MS diagnosed in 1994 who has been in complete remission since a dose of 3800 mg of cyclophosphamide was accidentally given intravenously in early 1997. For 7 years there have been no signs of disease activity on history, physical examination, or on high-quality magnetic resonance imaging (MRI) with appropriate contrast-enhancement methodology. This case includes information on the possibility that less aggressive chemotherapy than that used with stem cell transplantation may be effective in the long-term control of MS.