Delirium -- sudden, severe confusion that occurs with acute illness -- is very common in ICU patients. As a group, patients experiencing delirium in the ICU tend to have worse outcomes than patients without delirium, all other things being equal (as far as that equality can be determined). But no one knows if it is delirium itself that worsens outcomes from critical illness, or if delirium is simply a marker of worse illness or vulnerability in the patient (i.e., an unmeasured, confounding variable -- the whole