In a 32-year-old man with left-sided pneumothorax, dyspnoea and tachypnoea with a drop in blood pressure and severe hypoxaemia recurred during drainage. The chest X-ray revealed an alveolar infiltrate in the middle and lower segments of the left lung that was interpreted as re-expansion pulmonary oedema. The patient was respirated and given extra fluids. Extubation was possible after four days. Re-expansion pulmonary oedema is a rare, potentially lethal complication after treatment of a collapsed lung due to a hydro- or pneumothorax. The oedema usually occurs in the re-expanded lung and must be considered when there is a sudden respiratory or haemodynamic deterioration after drainage of X fluid or air. The diagnosis is made by clinical examination and chest X-ray. Predisposing factors are a large hydro- or pneumothorax, long-term collapse, young age and rapid re-expansion. This complication can be prevented by low negative suction pressures and by aspirating a maximum of 1000 to 1500 ml of pleural fluid. The pathophysiology of re-expansion pulmonary oedema is not completely understood and is probably multifactorial. The treatment is merely supportive: oxygen, intravenous fluid substitution and if necessary artificial ventilation. The use of diuretics should be avoided in patients with hypotension.