Recent studies indicate that oxidative stress caused by Helicobacter pylori and insufficient host antioxidant defense could play important role in pathogenesis of gastrointestinal ulcerations. By specific monoclonal antibodies we have detected weak presence of the major lipid peroxidation bioactive marker 4-hydroxynonenal (HNE) in healthy human gastric mucosa, which strongly increased in case of H. pylori-associated peptic ulcer. Considering physiological presence of HNE on one hand, and high prevalence of H. pylori associated disorders on the other, evaluation of oxidative stress after treatment is important. Therefore, in current study immunohistochemical accumulation and distribution of HNE-protein adducts in gastric mucosa was evaluated with 21 patients having H. pylori-associated duodenal peptic ulcer (DPU) before and one month after eradication of H. pylori. Although dramatic decrease in histological manifestations of inflammation was demonstrated after eradication of H. pylori, initially high immunopositivity for the HNE-protein adducts remained elevated in antrum and even increased in stomach corpus. The observed accumulation and redistribution to higher grades of HNE-immunopositivity in nuclei of glandular cells in gastric corpus indicate augmentation of oxidative stress after treatment and open possibilities for adjuvant antioxidant treatments to protect gastric mucosa from progressive oxidative stress after eradication of H. pylori infection.