Wernicke's encephalopathy is an acute neurological disorder, caused by thiamine deficiency, which is clinically characterized by a triad of ophthalmoplegia, ataxia and disturbances of consciousness. Most frequently, the encephalopathy is a consequence of chronic alcoholism, but it may occur in other forms of malnutrition or malabsorption, such as in prolonged parenteral nutrition without the addition of thiamine, total gastrectomy, gastrojejunostomy, severe anorexia or hyperemesis gravidarum. Thiamine requirement increases during critical illness and in chronically depleted patients, and its supplementation may influence the outcome of the disease. We report a case of a 73-year-old man, who presented with recurred colon cancer, who was affected by Wernicke's encephalopathy while undergoing oral feeding and parenteral nutrition. This case suggests that we should be more aware of potential thiamine deficiency in critically ill patients, even those undergoing parenteral nutrition and temporary oral feeding.