Respiratory failure is one of the most important causes of death in patients with acute pneumococcal pneumonia. There are two forms that may or may not coexist: ventilatory failure and hypoxemic respiratory failure. Ventilatory failure is principally caused by mechanical changes in the lungs resulting from pneumonia. Inflammatory exudate fills alveoli at slightly less than their normal functional residual capacity (FRC), causing a volume loss at FRC roughly proportional to the extent of the pulmonary infiltrate. Because this consolidated air space does not inflate easily at higher transpulmonary pressures, at higher lung volumes the volume loss is proportionally greater. This loss of volume reduces total lung compliance and increases the work of breathing. There is also evidence that the dynamic compliance of the remaining ventilated lung is reduced in pneumococcal pneumonia, possibly by reduction in surfactant activity, further increasing the work of breathing. Arterial hypoxemia early in acute pneumococcal pneumonia is principally caused by persistence of pulmonary artery blood flow to consolidated lung resulting in an intrapulmonary shunt, but also, to a varying degree, it is caused by intrapulmonary oxygen consumption by the lung during the acute phase and by ventilation-perfusion mismatch later. The persistence of pulmonary blood flow to consolidated lung appears to be caused by a relative failure of the hypoxic pulmonary vasoconstriction (HPV) mechanism during acute pneumonia, which is at least caused by endogenous vasodilator prostaglandins associated with the inflammatory process but also by other as yet undefined mechanisms. During convalescence, arterial oxygenation improves as blood flow to consolidated lung falls. The magnitude of the intrapulmonary shunt may be influenced by a number of factors that modify the distribution of pulmonary blood flow. Factors that tend to increase flow to consolidated lung and worsen shunt include endogenous vasodilator mediators, exogenous systemically administered vasodilator drugs, positioning the patient with the affected lung dependent, and increasing positive airway pressure. Factors that tend to reduce shunt include effective HPV, inhaled locally acting vasodilators that act primarily on ventilated lung, and positioning the patient with the affected lung up. Although thoughtful application of what is known about the pathophysiology of the lung in pneumococcal pneumonia can help the clinician deploy most effectively the available technologies of respiratory support in these patients, even the best intensive supportive measures are frequently inadequate, and mortality rates for patients requiring such support remain unacceptably high.