Wernicke's encephalopathy is a common complication of thiamine deficiency among chronic alcoholics. However, there have been few reports about MR imaging findings, including the diffusion-weighted changes of this neurologic disorder, in nonalcoholic patients. We present here a rare case of acute Wernicke's encephalopathy that developed in a patient who received prolonged total parenteral nutrition for his pseudomembranous colitis. The MR imaging, including the diffusion-weighted imaging, was performed at the onset of disease and during follow-up. The diagnosis was made by the characteristic MR imaging findings and it was supported by the clinical features. The initial and follow-up MR imaging findings with diffusion-weighted imaging changes are described and correlated with the clinical status.