Acute Wernicke's encephalopathy (WE) is caused by profound vitamin B1 (thiamine) deficiency and commonly presents with the classic clinical triad of mental confusion, ataxia, and ophthalmoplegia. This characteristic presentation results from the propensity of acute thiamine deficiency to preferentially injure specific brain regions: the dorsomedial thalamus, periaqueductal gray, and mamillary bodies. In these regions, abnormal magnetic resonance signaling on conventional sequences has been well described; however, diffusion restriction has only recently been reported. The authors demonstrate diffusion-weighted imaging (DWI) abnormalities of the splenium of the corpus callosum in a patient with acute WE, which has not been reported previously, and suggest a potential pathological mechanism. With the recent addition of DWI, MRI is becoming more sensitive to the changes in acute WE. Furthermore, the use of apparent diffusion coefficient mapping to evaluate the extent of likely underlying cytotoxic injury may help determine long-term response to vitamin therapy and, thus, disability.