Half of the world's population is infected with Helicobacter pylori, making it a pathogen of potentially great significance. Although in the majority of cases, infection is harmless, many infected individuals develop peptic ulcer disease or gastric cancer. In developed countries, the prevalence of infection has decreased along with the incidences of duodenal and gastric ulcer and distal gastric cancer. In developing countries, H. pylori infection rates remain high, and, as life expectancy increases, the incidence of gastric cancer will increase. Recently, a working party of the World Health Organization designated H. pylori a class I carcinogen. This designation, along with the description of an H. pylori-infected animal model of gastric cancer, has strengthened the case for H. pylori eradication to prevent gastric cancer. However, concerns are growing that H. pylori eradication itself may cause harm. And, in developed countries, an increase in the rate of cancers arising near the gastroesophageal junction might be linked to the disappearance of H. pylori. Therefore, we are faced with a dilemma: Should we eradicate H. pylori to prevent cancer of the distal stomach, or should we leave it and hence avoid cancer of the proximal stomach or distal esophagus? The need for more reliable treatments further complicates the issue. So far, we cannot assume that current treatment regimens provide adequate eradication rates to justify widespread screening. Further research is needed to determine in whom, when, and how to eradicate H. pylori infection.