It is recognized that diffusion-weighted magnetic resonance imaging is a sensitive method of detecting cerebral lesions in various neurologic disorders. This report presents two patients with acute encephalitis or encephalopathy who manifested similar serial changes on diffusion-weighted magnetic resonance imaging. Clinically, Patient 1, a 2-year-old male, was diagnosed as having hemiconvulsion-hemiplegia-epilepsy syndrome and Patient 2, a 9-month-old male, acute encephalitis associated with exanthema subitum. Despite the different etiology and the distribution of lesions, diffusion-weighted magnetic resonance imaging of these two patients revealed high-intensity lesions in the subcortical white matter in the acute phase, and then in the cortex, or basal ganglia, or both. In the convalescent phase, high-intensity lesions disappeared and brain atrophy developed. These serial changes were not recognized using other conventional methods. Although the exact mechanism for these serial changes remains unknown, these changes might reflect some pathogenic mechanism in acute encephalopathy or encephalitis.