Mechanical ventilation may have adverse effects on the lung. The appearance of extra-alveolar air, either as a pneumothorax or as subcutaneous emphysema along with other manifestations, is a complication of barotrauma which has been known for a long time. Recent experimental studies have clearly shown that mechanical ventilation can also lead to alterations in the blood gas barrier. Mechanical ventilation with high inflation pressure and elevated tidal volumes induces pulmonary oedema; the genesis of which results principally from anomalies of alveolar capillary permeability. These anomalies are made as a result of pulmonary distension and not as a result of elevated pressures in the airways, thus justifying the term "volume traumatism". The existence of previous acute pulmonary injury considerably worsens the deleterious pulmonary effect of mechanical ventilation. Although the direct clinical implications of these experimental studies are difficult to confirm, these latter have nevertheless lead to profound changes in ventilatory strategy during the course of acute pulmonary disease such as the adult respiratory distress syndrome.