Acute respiratory insufficiency (ARI) (reduced PaO2 and/or increased PaCO2) in surgical patients is mostly caused by atelectasis or bronchopneumonia. These complications may develop if functional residual capacity (FRC) is reduced. Even in surgical patients with normal lungs the supine position, some pulmonary disorders after any general anaesthesia, pain, and atonic intestine cause reduced FRC in the postoperative phase. Successful prophylaxis is based on these mechanisms, which are described. The adult respiratory distress syndrome (ARDS, termed "shock lung" a few years ago) is the most dangerous pulmonary complication leading to ARI. The evaluation of clinical data sugggests that noxious factors only active for a short time (so called "triggers") may start the development of ARDS when some physiological conditions (so called "constellations"), such as low flow syndrome, reduced FRC and overhydration, pave the way for this. In the early state prognosis is good if the patient is ventilated (CPPV) and kept on the dry side, and if cardiac output is elevated compared to the normal value at rest. Once fully established, ARDS has such a poor prognosis that the need for earliest possible commencement of therapy must be stressed emphatically. The following values may be taken as clear and simple symptoms for early diagnosis: vital capacity below 15 ml/kg (reflects decreased FRC), PaO2 breathing spontaneously room air below 60 mm Hg or 8 kPa (reflects increased intrapulmonary right to left shunt or regional hypoventilation), and respiratory rate above 25/min (reflects loss of compliance).