Spontaneous respiration in the presence of upper airway obstruction causes considerable negative intra alveolar pressure which may lead to pulmonary oedema "ex vacuo". Four cases are presented of spontaneously breathing patients who sustained upper airway obstruction lastin from one to several hours, leading to manifest pulmonary oedema. The pathogenesis of pulmonary oedema ex vacuo is discussed on the basis of alteration of physiological parameters such as capillary, alveolar and pleural pressures, as well as the properties of lung liquid exchange. In contrast, pulmonary oedema occurring after re-expansion of a collapsed lung is reported in one patient. No negative intraalveolar pressure could be incriminated in this case since the patient was ventilated using intermittent positive pressure from the beginning of lung expansion. We tend to attribute the evolution of this second kind of pulmonary oedema to capillary damage, resulting from hypoperfusion of the atelectatic areas, altered alveolar surface lining layer, infection and other cases. The therapeutic measures used in pulmonary oedema "exvacuo" are briefly mentioned.