Participants will include people on mechanical ventilation who are enrolled in either the ARDSNet 06 or 07 studies. For this study, participants will undergo blood collection within 48 hours of beginning the main ARDSNet study. There will be no study visits specifically for this study. Study researchers will analyze participants' blood samples and ARDSNet study data to gather information on mortality, the number of ventilator-free days, markers of severity of organ dysfunction and inflammation, and neutrophil response.