Acute respiratory distress syndrome (ARDS) is a type of respiratory failure characterized by rapid onset of widespread inflammation in the lungs. Symptoms include shortness of breath (dyspnea), rapid breathing (tachypnea), and bluish skin coloration (cyanosis). For those who survive, a decreased quality of life is common.

Causes may include sepsis, pancreatitis, trauma, pneumonia, and aspiration. The underlying mechanism involves diffuse injury to cells which form the barrier of the microscopic air sacs of the lungs, surfactant dysfunction, activation of the immune system, and dysfunction of the body's regulation of blood clotting. In effect, ARDS impairs the lungs' ability to exchange oxygen and carbon dioxide. Adult diagnosis is based on a PaO₂/FiO₂ ratio (ratio of partial pressure arterial oxygen and fraction of inspired oxygen) of less than 300 mm Hg despite a positive end-expiratory pressure (PEEP) of more than 5 cm H₂O. Cardiogenic pulmonary edema, as the cause, must be excluded.

The primary treatment involves mechanical ventilation together with treatments directed at the underlying cause. Ventilation strategies include using low volumes and low pressures. If oxygenation remains insufficient, lung recruitment maneuvers and neuromuscular blockers may be used. If these are insufficient, extracorporeal membrane oxygenation (ECMO) may be an option. The syndrome is associated with a death rate between 35 and 46%.

Globally, ARDS affects more than 3 million people a year. The condition was first described in 1967. Although the terminology of "adult respiratory distress syndrome" has at times been used to differentiate ARDS from "infant respiratory distress syndrome" in newborns, the international consensus is that "acute respiratory distress syndrome" is the best term because ARDS can affect people of all ages. There are separate diagnostic criteria for children and those in areas of the world with fewer resources.

The signs and symptoms of ARDS often begin within two hours of an inciting event, but have been known to take as long as 1–3 days; diagnostic criteria require a known insult to have happened within 7 days of the syndrome. Signs and symptoms may include shortness of breath, fast breathing, and a low oxygen level in the blood due to abnormal ventilation. Other common symptoms include muscle fatigue and general weakness, low blood pressure, a dry, hacking cough, and fever.

Complications may include the following:

Other complications that are typically associated with ARDS include:

There are direct and indirect causes of ARDS depending whether the lungs are initially affected. Direct causes include pneumonia (including bacterial and viral), aspiration, inhalational lung injury, lung contusion, chest trauma, and near-drowning. Indirect causes include sepsis, shock, pancreatitis, trauma (e.g. fat embolism), cardiopulmonary bypass, TRALI, burns, increased intracranial pressure. Fewer cases of ARDS are linked to large volumes of fluid used during post-trauma resuscitation.