To evaluate the incidence, pattern and clinical importance of endotoxemia in septic shock, frequent, serial endotoxin determinations were made prospectively in patients with shock. Detectable endotoxin occurred in 43 of 100 patients with septic shock, but in only one of ten patients with shock due to nonseptic causes. During septic shock, endotoxemia frequently occurred in the absence of Gram-negative bacteremia. Using a logistic regression model, multiple organ failure occurred 10.3 times more frequently and depression of left ventricular ejection fraction (less than or equal to 45 percent) 4.8 times more frequently in endotoxemic patients. In patients with positive blood cultures, endotoxemia was associated with a high mortality. We conclude that endotoxemia occurs frequently in septic shock and is associated with severe manifestations of this syndrome, including cardiac depression and multiple organ failure. This study suggests that endotoxin is an important mediator of septic shock and supports efforts to develop anti-endotoxin therapies for treating patients with this disease.