Purpose: The risk of H. pylori (HP) acquisition in individuals residing for prolonged time periods in endemic areas remains unknown. We investigated HP status in young Mormon adults who had served missions in developing countries for prolonged time periods to assess environmental influences on HP infection rates. Their HP status was compared to age-matched groups of (1) Mormon adults who had served in developed countries and (2) a control group of Mormon adults who had not left their home residences. We hypothesized that serving as a missionary in developing countries may confer an increased risk for HP acquisition. Methods: Serologic testing was used to assess HP status in 780 young missionaries (age 22.6 yrs, 2.3 SD; 480 males), of whom 236 had not left their home residence. 239 had served their missions in a developing country while 305 had served their missions in a developed country. Each enrollee completed the validated GSRS questionnaire for GI symptoms in addition to providing details of family size and birth order. The influences of various risk factors on HP status were expressed as odds ratio with their 95% confidence interval. Results: Of all subjects tested 5.4% harbored HP. The rate of infection was not different among subjects who had not left their home areas and in those who had returned from a mission. The infection rate was similar in those who had returned from a mission in a developing as opposed to a developed country. Compared with females, males harbored an increased risk for HP infection (2.39, 1.13-5.06). Regular consumption of antacids was associated with an increased risk for HP (2.01, 1.17-3.43). A high birth order was also borderline predictive for a positive HP status: 3.6 (2.0) versus 2.9 (1.8), t=1.90, p=0.0583. Other types of travel, age, family size, and GI symptom score exerted no significant influence. In a multivariate logistic regression analysis, use of antacids and birth order both remained statistically significant outcome predictors, whereas gender and none of the other predictor variables contributed significantly to the overall model. Conclusion: Among young Mormon adults, serving a mission for a prolonged time period in a developing country did not confer an increased risk for HP infection. Conditions of childhood as affected by birth order and gender seem to exert a stronger influence on HP infection than subsequent events during early adulthood. Increased antacid consumption among carriers of HP infection may influence dyspeptic symptoms in these subjects.