Aim: Increased oxidative stress is a significant part of pathogenesis of various lung diseases. In this trial, it is aimed to determine the role of oxidative stress in patients receiving mechanical ventilation for respiratory failure (RF). Methods: The oxidative stress was evaluated by determining plasma, bronchial fluid and erythrocyte levels of malondialdehyde (MDA), superoxide dismutase (SOD), glutathione peroxidase (GSH-Px), ascorbic acid, alpha-tocopherol, uric acid (UA) and nitric oxide (NO) in 25 critically ill patients with RF. Twenty five patients without RF was evaluated as control group. Results: Oxidative stress (MDA, NO) levels of plasma, erythrocyte and BAL fluid in mixed critically ill with respiratory failure were higher than control group. Antioxidant levels of plasma, erythrocyte and BAL fluid in mixed critically ill with respiratory failure were lower than control group. In all parameters, there was no statistically difference as exitus and survivors in critically mixed patients. Conclusion: Oxidative stress was higher in respiratory failure than control group.