A rare complication after delayed re-expansion of pneumothorax is reported. A polytraumatized patient with stable vital functions was admitted to our ICU immediately after surgery. Later, oxygenation worsened treated by a rise in FiO2. Concomitant tachycardia was thought to be due to increasing body temperature. On day 3 of treatment in the ICU further deterioration in gas exchange (and in hemodynamics, with complete collapse of the left lung) was diagnosed on X-ray examination. Retrospectively, the development of this condition could be traced on the X-ray films taken during the previous 3 days. Thoracic drainage and suction resulted in complete re-expansion of the lung. After re-expansion worsening of gas exchange and unilateral ARDS-like configurations were observed on chest X-ray. Reversal of the I:E ration and a rise in PEEP improved gas exchange and the X-ray appearance immediately. In the next few days the intensity of the respiratory treatment could be reduced, and after a short period of CPAP the patient was discharged from the ICU. Three mechanisms for development of this "unilateral ARDS" are discussed: loss and suppressed regeneration of surfactant in prolonged atelectic alveolar compartments; increased capillary fluid escape due to suction; and increased complement activation and reduced degradation of edematogenic bradykinin in hypoxic alveolar compartments. Possible clinical implications for the treatment of longer duration pneumothorax are: fractionated drainage and respirator settings, reopening collapsed alveoli in an inhomogeneously diseased lung such as IRV.