About one-third of the patients who develop mild, acute confusion (i.e., subsyndromal delirium) will go on to develop a severe acute confusional state (i.e. delirium). Delirium refers to a temporary change in the way a person thinks about things. Delirium occurs in patients admitted to the hospital particularly those patients that are very sick, who are given a number of medications, and who are not able to sleep normally. It affects their behavior, their understanding of the people and things around them, and their ability to make decisions. While ICU doctors do everything possible to eliminate the factors that may cause delirium, delirium may cause a person to become very agitated which if not controlled is dangerous to their safety as well as the safety of those around them. As well, if delirium develops in patients in the ICU, it may increase the risk for death, keep patients in both the ICU and hospital for longer and send patients to a long term care facility rather than home after they are discharged from the hospital. A recent medical report found that patients in the ICU who develop subsyndromal delirium have a mortality rate, a length of stay in both the ICU and the hospital, and a transfer rate to a long term care facility that is nearly as great as patients with delirium and greater than patients who develop neither subsyndromal delirium or delirium. Recent studies in non-ICU patients suggest that if a patient who is at high risk for developing delirium receives a medication called an antipsychotic (e.g. haloperidol) they may not be as likely to develop delirium or if they do develop delirium it will not last as long. No studies have evaluated the effect of administering an antipsychotic in patients in the ICU who have subsyndromal delirium. Another study completed in the ICUs at Tufts Medical Center found that there may be an association between the development of delirium in patients with subsyndromal delirium and the use of haloperidol. However, this small study had many limitations and thus it is not currently known whether using haloperidol in patients with subsyndromal delirium will actually cause fewer of these patients to develop delirium. Haloperidol makes a person sleepy and helps control behavior like agitation. Haloperidol is the drug that is used most often to help control delirium in the ICU. This prospective, double-blind, randomized controlled study will determine if haloperidol administered through the vein four times daily (1mg IV q6h) to patients who have subsyndromal delirium, and who are on a breathing machine and being cared for by the Medical ICU service at Tufts Medical Center, will help prevent patients from developing delirium. A total of 68 participants will be enrolled. Exclusion criteria are extensive and include conditions that could affect the ability to determine if delirium is present or increase the risk for side effects related to the administration of haloperidol. Patients older than 80 will be excluded from the study. Study medication (i.e. haloperidol) will be administered until one the following occurs: 1) delirium develops (that is confirmed by a staff psychiatrist or his designate, 2) the patient is discharged from the ICU at Tufts Medical Center, 3) the patient has received haloperidol or placebo for 10 days or 4) an adverse event potentially attributable to the study drug is experienced by a patient that is deemed, in the opinion of a pulmonologist member of the investigative team to warrant discontinuation of therapy. Haloperidol may cause unwanted side effects such as low blood pressure, twitching, and an unsafe abnormal heart rhythm. Patients with chronic confusion (e.g., a dementia such as Alzheimer's Disease) should not receive haloperidol and will not be included in this study. Patients will be carefully monitored for side effects that are potentially related to haloperidol. Patients who become confusion-free in the ICU before they leave the ICU (i.e., have no subsyndromal delirium) will be asked to provide consent for all research activities that occured in the ICU. If patients where cognition is regained (ie. no subsyndromal delirium or delirium) are not willing to provide consent then any study data collected from them while they were in the ICU will be destroyed and they will not be approached to participate in the post-ICU component of the study. This study also seeks to understand how the use of haloperidol in the ICU in patients with subsyndromal delirium may have affect memory, emotional status, happiness, ability to function, and quality of sleep in patients after they leave the ICU. Patients (that do not have delirium based on CAM screening at the time the 3-10 day and 6 month assessments are attempted) will be approached to participate in this post-ICU component of the study.