the management of Ventilator-associated pneumonia (VAP) caused by multidrug-resistant (MDR) gram-negative bacilli (GNB) represent a real therapeutic dilemma in intensive care unit (ICU). Colistin remains an effective agent against MDR GNB. However, because of its side effects, mainly nephrotoxicity, other modalities than the intra venous (IV) route should be tried. Several recent data emphasize the interest of inhaled route. The investigators purpose was to evaluate the effectiveness and systemic toxicity of aerosolized colistin in ventilator associated pneumonia.