Wallerian degeneration (WD) after ischemic stroke has been associated to persistent motor impairment, but signal intensity changes on conventional magnetic resonance imaging (MRI) are generally not detected until four weeks after the event. We report a 54 year old male patient, referred to our hospital for sudden-onset left hemiparesis. Cerebral CT showed right fronto-parietal infarct (middle cerebral artery stroke). We performed two CT control, which revealed no haemorrhagic transformation. MRI, obtained 13 days after the onset, demonstrated the infarct, mainly subcortical, extending throughout fronto-temporo-parietal areas and restricted diffusion in the ipsilateral corticospinal tract. In conclusion, WD is apparent on diffusion-weighted imaging within two weeks of stroke, allowing a better prognostic evaluation of recovery. The abnormal signal should not be misinterpreted as new ischaemic lesions.