The production of ROS in the gastrointestinal tract (GI) and their role in the pathophysiology and pathogenesis of gastrointestinal diseases have not been studied sufficiently. In the plasma of patients, in the context of the sequence gastro oesophageal reflux-oesophagitis-metaplasia-dysplasia-adenocarcinoma, have been found simultaneous formation of DNA adducts and increased myeloperoxidase concentration, which are associated with oxidative stress, decreased antioxidant capacity (decreased glutathione concentration).These findings support the role of oxidative stress in the pathogenesis and malignant transformation.