Intensive medical and surgical support of organ systems has improved dramatically in the past 20 years, making salvage from previously unsurvivable illnesses and injuries possible. As a consequence, deep-seated infection and associated progressive respiratory failure have become the major soure of morbidity and mortality among critically ill patients. Recently, several investigations have focused attention upon methods designed for early detection and subsequent followup of pulmonary dysfunction. These studies suggest that early detection of pulmonary dysfunction and institution of therapeutic modalities may be associated with fewer serious pulmonary complications. Sequential evaluation of oxygenation is the most readily available method for following pulmonary dysfunction in this critically ill patient population.