Probiotics are commercially available live bacteria thought to have health benefits when ingested. A literature review of probiotic studies in the intensive care unit (ICU) found that in patients who receive probiotics, there is a 25% reduction in lung infection, known as ventilator-associated pneumonia (VAP). There is also an 18% reduction in the chance of developing any infection in the ICU. However, the studies reviewed were small and not well done. Therefore, whether probiotics are really helpful or not is unclear. Before a large carefully performed study is done to evaluate the effects of probiotics in critically ill patients, a pilot trial is needed. The investigators plan to study the feasibility of such a trial in mechanically ventilated ICU patients in 11 ICUs in Ontario, investigating whether orally ingested L. rhamnosus GG (a common probiotic) prevents VAP and other infections. The 4 goals of the pilot study will be to ensure that we can successfully enrol patients; follow the protocol faithfully; make sure patients don't receive additional probiotics, and estimate how much pneumonia exists in these patients. In a separate substudy, the investigators will evaluate the effects of probiotics on lower lung infiltration with potential harmful bacteria, stool bacteria, markers of immunity called cytokines, and markers of infection called endotoxin levels. The investigators plan to enroll 150 adults admitted to ICU and receiving mechanical ventilation. Following informed consent, patients will be randomized to either L. rhamnosus GG group or an identical placebo. Twice daily, patients will receive probiotics or placebo in a feeding tube. The investigators will record all infections and other important outcomes in the ICU. This study is very important in the ongoing search for more effective strategies to prevent serious infection during critical illness. Probiotics may be an easy-to-use, readily available, inexpensive approach to help future critically ill patients around the world.