Delirium occurs in up to 80% of mechanically ventilated patients admitted to the intensive care unit (ICU). (1-4) Patients who develop delirium are more likely to die, stay in the ICU longer and cost the healthcare system more. (1-6) Moreover, delirium may cause cognitive dysfunction that persist long after recovery and impact long-term functional ability and quality of life. (7-10) Given that more than 5 million patients are admitted each year to an ICU in the United States, delirium in the critically ill is a major health care issue. Patients who meet some but not all of the Diagnostic and Statistical Manual of Mental Disorders (DSM) IV criteria for delirium are deemed to have subsyndromal delirium. (19) Patients who develop subsyndromal delirium are also more likely to die in the ICU and require a higher level of care after ICU discharge. (19) Current practice guidelines recommend routine delirium screening in the ICU using a validated tool such as the Intensive Care Delirium Screening Checklist (ICDSC). (14, 22) Delirium often persists despite the reversal of causative factors thereby necessitating the use of pharmacologic therapy. Haloperidol remains the pharmacologic treatment of choice for delirium in the ICU and its use has been associated with lower ICU mortality. (23) However, its potential role in preventing the conversion of subsyndromal delirium to delirium is unknown. In this study we plan to determine whether the treatment of subsyndromal delirium in the critically ill with haloperidol prevents progression to delirium without causing adverse effects.