Accomplishing these aims will determine if a pharmacologic intervention, haloperidol, can be routinely used in critical care practice to prevent patients with subsyndromal delirium from developing clinical delirium. If haloperidol is found to be safe and effective in the proposed study, larger studies will be needed to determine the effect of haloperidol therapy on patient mortality and evaluate predictors for drug response. Information from the proposed study is greatly needed to determine if there is a safe and effective pharmacological intervention for the estimated 1.5 million Americans each year who develop subsyndromal delirium in the ICU.