To evaluate the usefulness of diffusion-weighted MRI (DWI) for the early diagnosis of Creutzfeldt-Jakob disease (CJD). Thirty-six consecutive patients (age 56 to 82 years) were enrolled, and 26 were examined by DWI. Nine were definite based on the World Health Organization criteria, and 27 were probable. The percentages of DWI abnormalities, periodic sharp wave complexes (PSWCs) on the EEG, detection of CSF 14-3-3 protein, and increase of CSF neuron-specific enolase (>25 ng/mL) on the first examination were compared. For DWI, 32 patients (age 31 to 84 years) who showed progressive dementia or impaired consciousness served as disease controls. The percentage of DWI abnormalities was 92.3%, of PSWCs 50.0%, of 14-3-3 protein detection 84.0%, and of NSE increase 73.3%. Two of the 32 control subjects were falsely positive on DWI. The sensitivity of DWI was 92.3% (95% CI 74.8 to 99.5%) and specificity 93.8% (95% CI 79.2 to 99.2%). In 17 patients who did not show PSWCs on the first EEG, abnormal DWI findings were still clearly detected. Four patients who were negative for 14-3-3 protein also showed DWI abnormalities. DWI abnormalities were detected as early as at 3 weeks of symptom duration in four patients in whom PSWCs were not yet evident. DWI can detect characteristic lesions in the majority of patients with CJD regardless of the presence of PSWCs. DWI was the most sensitive test for the early clinical diagnosis of CJD; consideration should be given to its inclusion in the clinical diagnostic criteria of CJD.