PURPOSE: This study will examine the association between delirium and thiamine levels, plus environmental and genetic pre-disposition to thiamine deficiency in intensive care unit (ICU) patients who are mechanically ventilated for longer than 48 hours. This study is highly relevant to the mission of the National Institute of Nursing Research (NINR), specifically to understanding causative mechanisms of symptoms of delirium. BACKGOUND: Patients in the ICU are at high risk of delirium due to illness severity, older age, sepsis and sedative-opiate drugs. ICU patients are also risk thiamine deficiency for a variety of reasons including: diuretic use, increased metabolic rate, sepsis, hyperglycemia, preexisting diabetes, preexisting alcohol abuse, inadequate nutritional intake, and nutritional loss due to diarrhea or vomiting. AIMS: The specific study aims are to: 1. examine the relationship between delirium and thiamine levels over time;2. examine the relationship between pre-ICU risk factors for thiamine deficiency and admission thiamine levels;and 3. test two candidate genes for their association with delirium and thiamine levels, in critically ill mechanically ventilated patients. SAMPLE: 122 patients, who are mechanically ventilated for longer than 48 hours in the ICU. DESIGN &METHOD: A longitudinal research design is used for this study. Eligible patients will be recruited within 48 hours of ICU admission in order to obtain a baseline thiamine level. Measurement of thiamine level and clinical assessment for presence of delirium will occur daily to a maximum of ten days. RELEVANCE TO PUBLIC HEALTH: Delirium is a frequent finding in ICU patients. It is a distressing symptom for affected patients and more information about the causes may lead to more effective treatments. This study is designed to examine whether a metabolic disturbance such as thiamine insufficiency, combined with a genetic predisposition toward either thiamine deficiency or delirium, increases the likelihood of developing symptoms of delirium in critically ill patients.