Selenium has a double action. (i) Seleno-compounds, among them sodium selenite have a direct pro-oxidant action leading to acute toxicity but may be also beneficial as drug. (ii) Selenium is an essential anti-oxidant required for anti-oxidant seleno-enzymes. Septic shock is a common severe syndrome leading to endothelium damage and multiple organ failure, with increased data suggesting the principle role of oxidative stress. Selenoprotein P, main selenium constituent of the plasma, may decrease dramatically and specifically in septic shock patients and may be involved in the endothelium protection. A prospective, multi-center placebo-controlled, randomized, double-blind study in severe septic shock patients with documented infection has been preformed. Patients received, for 10 days, selenium as sodium selenite (4000 microg on the first day, 1000 microg/day on the 9 following days) or matching placebo using continuous intravenous infusion. Mortality rates did not significantly differ between groups at any time point. Adverse events rates were similar in the two groups. However, high-dose selenium administration has been associated with a tendency to decrease the mortality in septic shock animal and patients, especially when using a bolus administration, whereas studies using a continuous administration failed to find any benefit on mortality. The interest of the successive use of pro-oxidant action of seleno-compounds, followed by anti-oxidant action need to be the further studied in cellular and animal models, preceding new dose-effect phase II. The interest of the selenoprotein-P as a marker of septic shock and for endothelium protection needs also to be studied further.