Multiple sclerosis is the most common demyelinating disease of the central nervous system affecting young adults, in which destruction of the axon myelin sheath disturbs signal transduction. The disease course is usually remitting and relapsing, but sometimes there is steady neurological deterioration. The diagnosis depends mainly on an adequate clinical history and neurological examination. Evoked potentials, elevated cerebrospinal fluid gamma globulin with oligoclonal bands, and imaging studies, mainly magnetic resonance imaging (MRI), also contribute to the diagnosis. Multiple sclerosis may occasionally present as a mass lesion that clinically and radiologically is indistinguishable from a brain tumor. We present 2 cases of giant tumefactive lesions, proven by brain biopsy to be of demyelinating nature.