Research suggests that the forces exerted on the lungs by mechanical ventilators may cause as much damage to the lungs as the original pathologic process. In an attempt to limit additional injury to damaged lungs and improve the morbidity and mortality of patients requiring mechanical ventilation, investigators have proposed a controversial method of ventilatory management, permissive hypercapnia. This method attempts to maintain adequate oxygenation while allowing ventilation to decrease; carbon dioxide increases. The use of permissive hypercapnia is advocated in patients with acute lung injury and status asthmaticus. Ventilating pressures and volumes are lowered, with a resultant lower minute ventilation. Few adverse effects have been noted when this process has occurred gradually. By using permissive hypercapnia from the initiation of mechanical ventilation, it is possible to support the body through the resolution of the disease process while preventing additional lung injury.