The acute respiratory distress syndrome (ARDS) is a sudden, serious, and sometimes fatal condition. In ARDS, the lungs become inflamed and filled with fluid so that they do not absorb oxygen well. Many diseases and injuries can lead to ARDS, including pneumonia, breathing in vomited stomach contents, near-drowning, severe burns, and bloodstream infections. Clinical doctors usually use several criteria to diagnose ARDS, including chest x-ray findings and low levels of oxygen in the blood. Pathologists (doctors who diagnose diseases by examining tissue under a microscope) also use a specific pattern of tissue injury (diffuse alveolar damage) to diagnose ARDS. Few studies examine whether all patients who meet clinical criteria for ARDS actually have lung tissue findings that confirm that diagnosis.