We studied prospectively the quantitative relation of circulating endotoxin (lipooligosaccharides [LOSs]) and the development of multiple organ failure and death in 45 consecutively admitted patients with bacteriologically verified systemic meningococcal disease (SMD). A plasma LOS level of greater than 700 ng/L correlated with development of severe septic shock (P less than .0001), adult respiratory distress syndrome (P = .0035), a pathologically elevated serum creatinine level (P less than .0001), or death as a consequence of multiple organ failure (P = .0002). Initial plasma LOS levels of less than 25, 25-700, 700-10,000, and greater than 10,000 ng/L were associated with 0%, 14%, 27%, and 86% fatality, respectively. The LOS half-life after initiation of antibiotic therapy was 1-3 h. Increasing plasma LOS levels were never seen. These observations suggest that LOS quantitation using the limulus amebocyte lysate assay with a chromogenic substrate gives important progsnotic information and may provide new insight concerning pathophysiological aspects of SMD.