Acute respiratory distress syndrome (ARDS) is the complication of many acute conditions, which can be characterised with a non-cardiogenic lung edema and hypoxemia. The most common risk factors for developing ARDS can be grouped into direct (acute severe pneumonia, aspiration of gastric content) and indirect (sepsis, acute severe pancreatitis, massive blood component transfusion) effects (Galvin et al., 2011). The incidence of ARDS since 1990 has doubled (Sigurdsson et al., 2013), and the level of mortality can reach even 50% (Ferguson et al., 2007) in intensive care unit (ICU) patients. The treatment of these patients can be very complicated, and can include serious fluid management, feeding, extra corporal membrane oxygenation, and mechanical lung ventilation advanced techniques.Imbalance between oxidants and antioxidants may play an important role in pathogenesis of ARDS. The reactive oxygen molecules can be produced by all cells in the organism due to enzymatic and non-enzymatic processes. Lists of oxidative stress markers associated with development of many disorders are already known (Kumar et al., 2000;Bowler et al., 2003;Crimi et al., 2006), but their predictive role in ARDS is still underestimated. Reactive oxygen species initiate cellular tissue death by lipid peroxidation, and as a result also modulation of proteins and DNA.Oxidative stress related molecules, such as malondialdechyde and 4-hydroxinonenal, are important products of non-enzymatic reactions. On the other hand, protective function is provided by antioxidative molecules, for example, superoxiddismutase, glutathionperoxidase, and tocopherol.The main aim of this study was to investigate the dynamic changes of the level of oxidative stress markers in patients with acute respiratory distress syndrome. The secondary goal was to determine the relationship of the level of oxidative stress markers to the outcome in patients with acute respiratory distress syndrome.This prospective study was conducted in the ICU of Pauls Stradiòð Clinical University hospital during six months in 2013 according to the following inclusion criteria: mechanical lung ventilation 24 hours in patients over 18 years of age and acute severe pneumonia, pancreatitis, sepsis or massive blood component transfusion. The study was approved by the Ethics Committee of Rîga Stradiòð University.Patients with ARDS were monitored for seven days. The ARDS was diagnosed according to the Berlin definition (Ranieri et al., 2012)