Procalcitonin (PCT) is a biomarker which is elevated in the blood of patients with severe sepsis/septic shock. Data from patients with community acquired pneumonia demonstrated that this biomarker can be used to decide on the duration of antimicrobial therapy. Studies with small sample size seem to confirm this in ICU patients with severe sepsis. However, this needs to be confirmed in a larger cohort. All patients are randomly allocated to a PCT guided algorithm or a control group. In the PCT-guided group, PCT is measured at randomization, day 4, 7, 10, and 14. Depending on the PCT course, the protocol recommends to change, alter, or stop anti-infectious measures. In the control group, anti-infectious therapy is left to the discretion of the treating physician.