Seroprevalence of antibodies to Helicobacter pylori is generally higher in developing than in developed countries. The route of transmission of H. pylori is unknown but is most commonly assumed to be fecal-oral. Gastroenteritis in a person traveling to developing countries is often a marker of exposure to fecally contaminated food or water. Of 133 initially seronegative young Swedes traveling to developing countries for a total of 16.4 years, of whom 102 reported having had at least one episode of gastroenteritis, not one seroconverted. This rate is lower than in studies of residents in developed countries and casts some doubt on the theory of fecal-oral transmission via a common source as an important mode of transmission of infection with Helicobacter pylori.