Fever is a major event in critically ill patients. Its causes are difficult to identify and among them, bacterial infections must be rapidly identified and treated before sepsis reach a life threatening stage. CRP is a very sensitive marker of inflammation of whatever causes including sepsis. This is particularly crucial in immunocompromised patients. On the other hand, PCT has been extensively shown to be strongly and rapidly produced in bacterial infections only. Furthermore, PCT decreases very soon after bacterial resolution, and is thus a very useful in monitoring adequacy and efficiency of antibiotics. The utility of this marker in sever sepsis is now widely admitted.