We report a rare case having dementia with transient splenial lesion on MRI after hypoglycemia. A 75-year-old woman with type 2 diabetes was admitted with mental change after medication of a hypoglycemic agent. Initial serum glucose was 22 mg/dL. High signal intensity in the splenium on diffusion-weighted MRI was shown. After conservative management, she was gradually improved and splenial lesion was disappeared, but her cognitive impairment remained.