The purpose of this study is to develop a scoring system to allow doctors to accurately identify children on a mechanical ventilator who have bacterial pneumonia. Currently this diagnosis is very difficult to make, resulting in the overuse of antibiotics and the promotion of antibiotic-resistant bacteria in the pediatric intensive care unit (ICU). Four ICUs at 3 children's hospitals will participate. Study participants will include 150 children, ages 2 months to 17 years old who require mechanical ventilation, and in whom the bedside health care providers suspect bacterial pneumonia. Bacteria will be studied by sampling lung fluid through the breathing tube less than 12 hours after starting antibiotics, using a procedure known as "non-bronchoscopic-bronchoalveolar lavage (NB-BAL)." Participants may be involved in study related procedures for up to 31 days.