Purpose: In terminally ill patients with advanced cancer,it is recognized that delirium is reversible in 20-50% of the patients with it. Identification of its cause is vital to ensure the quality of life of the patients with delirium at the end of life. We would like to report a case of the advanced cervical cancer patient with delirium, successfully treated by intravenous administration of vitamin B1. Case: An 83-year-old woman, who was diagnosed the advanced cervical cancer with carcinomatous peritonitis, was admitted to Shizuoka Cancer Center Palliative Care Unit. Four days after the admission, she presented sleep-wake cycle disturbance, poor attention, poor concentration,and short-term memory loss, and these conditions were diagnosed with delirium. Vitamin B1 deficiency was suspected by normal examinations including laboratory results and head computed tomography except for the low level (19ng/ml) of vitamin B1. One week after starting intravenous administration of vitamin B1, the symptoms of delirium were improved. Conclusion: In this case, delirium by vitamin B1 deficiency developed even though having adequate oral intake (about 1,000kcal/day), indicating malabsorption of vitamin B1 due to hypoperistalsis and edema of the bowel. Advanced cancer patients can easily develop vitamin B1 deficiency due to inadequate oral intake, increased consumption of vitamin B1 and malabsorption of vitamin B1. Therefore,the examination of vitamin B1 deficiency is necessary for patients with delirium that cannot be specified. Palliat Care Res 2009; 4(2): 330-333