Malignancy-associated primary thiamine deficiency has been documented in several experimental tumors, clinical case reports, and in patients with fast growing malignancies. We report a terminally ill cancer patient who developed delirium. Close examination of the patient demonstrated that delirium was caused by thiamine deficiency, although she had been consuming an average of 990 cal/day for the past 3 weeks. Malabsorption or consumption by the tumor was considered the mechanism of thiamine deficiency. Early recognition and subsequent treatment resulted in successful palliation of delirium. In terminally ill cancer patients, clinicians must remain aware of the possibility of Wernicke's encephalopathy, when the patients develop unexplained delirium, even if the patient has been consuming adequate amounts of food. Early intervention may correct the symptoms and prevent irreversible brain damage, and the quality of life for the patient may improve.