A 42-year-old white man presented with cognitive impairment and behavioral changes followed by rapidly progressive motor and gait impairment. Magnetic resonance imaging revealed striking multifocal white matter signal change, areas of restricted diffusion, diffuse callosal signal change, and atrophy and hyperintensity of the corticospinal tracts. A broad range of etiologies warrant consideration in this case, including degenerative, vascular, inflammatory, metabolic, and neoplastic diseases.