A 48-year-old man suddenly developed clinically and electroencephalographically nonspecific dementia. On MRI sequences, only diffusion-weighted images (DWI) of the cortex were unequivocally pathological. Obvious atrophy and basal ganglia signal changes appeared only 9 months after the onset. Brain biopsy confirmed Creutzfeldt-Jakob disease (CJD). In rapidly progressive dementia, we recommend DWI for early diagnosis of CJD.