Acute respiratory distress syndrome is a devastating disease that causes substantial morbidity and mortality. Mechanical ventilation can worsen lung injury, whereas ventilatory strategies that reduce lung stretch, resulting in a "permissive" hypercapnic acidosis (HCA), improve outcome. HCA directly reduces nonsepsis-induced lung injury in preclinical models and, therefore, has therapeutic potential in these patients. These beneficial effects are mediated via inhibition of the host immune response, particularly cytokine signaling, phagocyte function, and the adaptive immune response. Of concern, these immunosuppressive effects of HCA may hinder the host response to microbial infection. Recent studies suggest that HCA is protective in the earlier phases of bacterial pneumonia-induced sepsis but may worsen injury in the setting of prolonged lung sepsis. In contrast, HCA is protective in preclinical models of early and prolonged systemic sepsis.. Buffering of the HCA is not beneficial and may worsen pneumonia-induced injury.