Mechanical ventilators are used to provide life support for patients with respiratory failure. But over the long term, these machines can damage the lungs, causing them to collapse and the partial pressure of oxygen in the arteries to drop to abnormally low values1. In conventional mechanical ventilation, the respiratory rate and volume of air inspired per breath are fixed, although during natural breathing these parameters vary appreciably2. A computer-controlled ventilator has now been introduced3 that can use noise to mimic this variability. We describe a conceptual model of lung injury in which the partial pressure of arterial oxygen is improved significantly by computer-controlled rather than conventional mechanical ventilation, in agreement with recent experimental data3.