The acute respiratory distress syndrome (ARDS) in adults is associated with a wide variety of precipitating factors, often not directly involving the lung, and has an associated mortality of 50-80%. ARDS is almost invariably associated with sepsis, either as an initiating factor or as a secondary complication, which increases the expression of a number of cytokines impacting upon several cellular systems. Specifically, activation of neutrophils sequestered in the pulmonary circulation by this process, causes the release of free radicals and reactive oxygen species (ROS), increasingly regarded as key substances modulating the endothelial dysfunction and disruption responsible for the principal clinical manifestations of the syndrome. Here we discuss briefly the pathophysiology of ARDS and its impact upon pulmonary vascular control; the biological origins of free radicals and other ROS involved, the mechanisms of their damaging effects, their contribution to the modification of pulmonary vascular control mechanisms in lung injury and possible therapeutic perspectives.