Plasma LPS were quantitated in 40 patients with bacteriological verified systemic meningococcal infection (SMD). Twenty-two patients (55%) demonstrated LPS greater than 25 pg/ml. Fourteen patients with initial plasma LPS levels greater than 700 pg/ml developed a severe septic shock, impaired renal function and extensive coagulopathy compared to 2 out of 26 patients with LPS levels below 700 pg/ml. Seven patients died due to the circulatory collapse and multiple organ failure all with initial LPS levels greater than 1,000 pg/ml. Initiation of antibiotic therapy did not result in further increase of plasma LPS concentration. LPS were cleared from the circulation with a half-life of 1.5-3 hours after initiation of antibiotic treatment with penicillin and chloramphenicol. The results suggest that plasma LPS are quantitatively related to the development of septic shock, multiple organ failure and death from these complications in SMD, making LPS measurement a direct prognostic marker.