To alert health professionals to the need for early detection and prevention of shock in critically ill patients. By describing the associations between intramucosal acidosis in the gut and multiple system organ failure, the author demonstrates how noninvasive measurement of gut intramucosal pH can be used to monitor the adequacy of tissue oxygenation in the splanchnic organs and predict splanchnic ischemia within minutes of its onset. Review and analysis of current medical literature on shock and organ failure, combined with the author's prior research and expertise in the areas of tissue oxygenation and tonometric monitoring in the critically ill. The presence of defective tissue oxygenation in splanchnic organs and in gut ischemia may be detected within minutes of its occurrence via measurements of intramucosal pH in the gut. Measurement of intramucosal pH, obtained noninvasively with an intraluminally located gastrointestinal tonometer, provides an absolute metabolic measure of the adequacy of mucosal oxygenation. The putative consequences of intramucosal acidosis and associated mucosal injury include nosocomial pneumonia, myocardial depression, sepsis from enteric organisms, multiple system organ failure, and death. Through the use of routine monitoring of the adequacy of gut mucosal oxygenation, ischemic mucosal injury and its putative consequences can be prevented, resulting in reduced frequency of multiple organ failure and improved outcome. (Crit Care Med 1993; 21:S103-S107)