A patient operated for toxic megacolon secondary to ulcerative colitis developed a Wernicke syndrome (thiamine deficiency) during the postoperative period despite the administration of the usually recommended doses of vitamin B1 during total parenteral nutrition (TPN) treatment. Vitamin B1 deficiency should be checked in order to evaluate the patients' nutritional condition before starting TPN, especially those suffering from severe chronic malnutrition. Routine administration of vitamin B1 in repletion doses may be reasonably proposed in order to avoid the development of a Wernicke syndrome which is potentially lethal in a short time if not recognized and corrected in time.