It is well established that pharmacokinetic and pharmacodynamic parameters of antibiotics are correlated with their clinical and microbiological effectiveness. However in ICU patients, pharmacokinetic parameters of antibiotics suffer great variations, and bacteria responsible for these infections are usually less sensitive to antibiotics, especially Gram negative bacilli (GNB). An important pharmacodynamic variability may occur at the initial phase of the antibiotic treatment, decisive for the infection's outcome.