In a prospective open study, 61 consecutive patients with terminal cancer admitted to the hospital underwent cognitive assessment using the Mini-Mental State Questionnaire three times a week between admission and discharge or death. Mini-Mental State Questionnaire score upon admission was 28 +/- 1.5 in patients who were discharged (N = 14), and 25 +/- 3 in patients who died in the hospital (N = 47, P less than 0.01). The forty-seven patients who died in the hospital presented a total of 66 episodes of cognitive failure (CF) that were defined as a score of less than 24 or a drop of greater than 30% in the score on the Mini-Mental State Questionnaire. Of these 47 patients, 39 (83%) presented CF an average of 16 days before death. Upon detection of CF, a complete medical examination, laboratory evaluation, computerized tomography of the brain if indicated by abnormal findings on medical examination, and a complete medication review were performed. The cause of CF could not be established in 37 (56%) cases. Drugs, sepsis, and brain metastasis were the most frequently detected causes and were present in 6, 4, and 4 cases, respectively. In addition, 22 episodes (33%) of CF improved (10 episodes spontaneously and 12 episodes as a result of treatment). Our findings suggest that CF is extremely prevalent during the last weeks of life and, consequently, informed consent for therapeutic or research procedures or resuscitation may be impossible to obtain reliably at that stage.