Acute lung injury is a clinical syndrome of rapid onset of acute respiratory failure. It represents a significant public health issue. Patients with acute lung injury require admission into critical care units for advanced life support and utilize considerable health care resources. Published epidemiological studies on acute lung injury in the last 20 years are difficult to compare because they used different definitions and length of time for evaluation. Less than five studies have collected information for an entire year, and none of them have evaluated the degree of oxygenation failure under standard settings. We will perform a one-year prospective audit of all patients admitted with acute lung injury in almost 40 ICUs in Spain.