Reversible T2 hyperintense signal abnormality in the corpus callosum, although frequently seen after diffuse brain injury, has not been well clarified. With some accumulated evidence, we report a case of diffuse brain injury in a 24-year-old man. Magnetic resonance imaging (MRI) demonstrated T2 hyperintense signals in the trunk and the splenium of the corpus callosum 12 days postinjury. Echo-planar diffusion-weighted imaging was also performed on the same day, which revealed decreased diffusion (hyperintense signals) in the same site and almost the same size as T2 hyperintense signals. T1-weighted images were normal. Neuropsychological examination of the patient did not show callosal syndrome, namely hemialexia, unilateral agraphia and unilateral apraxia. Repeat MRI on day 20 demonstrated a signal decrease of both T2-weighted images and diffusion-weighted images (DWI) in the lesion. Follow-up MRI at 6 months showed complete resolution of the T2 signal abnormalities and of the corresponding decreased diffusion. Considering that diffusion-weighted imaging showed transient decreased diffusion, the lesion in the corpus callosum indicated the existence of cytotoxic edema. Also, transient DWI hyperintensity, namely cytotoxic edema, in the trunk and the splenium of the corpus callosum does not necessarily reveal callosal deficits.