The rediscovery of spiral bacteria in gastric mucosa in recent years has given raise to new speculations concerning the pathogenesis of peptic ulcer disease. Due to the features and localisation of this bacterium it was named campylobacter pylori. Findings pointing to a pathogenetic role of this bacterium are few and not definitely settled. The frequency of campylobacter on the gastroduodenal mucosa, its short term survival in acidic environment, its extraordinary mobility and content of specific enzymes makes it likely that this bacterium exerts a noxious effect on the gastroduodenal mucosa. Moreover a local and systemic immune response caused by campylobacter has been reported. We performed ultrastructural investigations on duodenal mucosa in patients with active peptic ulcer disease. An ultrastructural localization of the bacterium in duodenal mucosa of these patients was possible in 40%. Campylobacter pylori is localized preferentially on metaplastic cells of the antrum type, is seen penetrating in the intercellular spaces and is found also included in vacuoles of epithelial cells. These findings raise the hypothesis that campylobacter pylori infection can lead to direct damage of duodenal mucosa.