Multiple sclerosis (MS) is a demyelinating disease that predominantly affects those aged in their twenties to their forties. We recently observed a case of rapidly progressive late-onset MS in an elderly patient with a large, single lesion on magnetic resonance imaging (MRI), which was difficult to distinguish from a brain tumor. This case is reported here with a review of the literature. A 69-year-old woman who presented with left hemiparesis was admitted to our institute. A tumor lesion was strongly suspected from the MRI finding of a large solitary lesion adjacent to the right lateral ventricle. Her left hemiparesis became rapidly worse, so we decided to perform stereotactic biopsy. Histopathological examination indicated that the patient had a demyelinating disease such as multiple sclerosis. Steroid pulse therapy was started after the operation and led to a marked improvement of the symptoms. With follow-up for more than 1 year after discharge, she has shown no sign of relapse. Late-onset MS should be suspected if edema or mass effect is not visualized around the lesion on MRI, bearing in mind that late-onset MS can have a progressive course in elderly patients.