This study used monoclonal antibody specific for 4-hydroxynonenal (HNE)-histidine to evaluate immunohistochemical distribution of HNE-protein adducts in gastric mucosa biopsies of 52 peptic ulcer patients (all positive for H. pylori) and of 20 healthy volunteers (eight positive and 12 negative for H. pylori). HNE-modified proteins were present in glandular epithelium in all subjects, both patients with duodenal peptic ulcer and healthy subjects. Hence, the presence of HNE did not appear to be related to the presence of H. pylori. However, in patients with duodenal peptic ulcer accumulation of HNE-protein adducts was frequently observed also in nuclei, while in the control group such subcellular distribution of HNE was not observed at all. This study shows physiological presence of HNE in human gastric mucosa, but also suggests its role in pathology of gastric dysfunction in duodenal peptic ulcer patients manifested by accumulation of HNE-protein adducts in particular in nuclei of gastric glandular epithelium.