Objectives: Cross‐sectional studies of children in developing countries show a high prevalence of Helicobacter pylori infection at 6 months of age, hut a decrease in the prevalence of infection hetween 1 and 5 yr of age. The decrease suggests a loss or clearance of infection, an uncommon finding in adults. Our ohjective in this study was to determine the longitudinal persistence of H. pylori infection in young childen. Methods: We tested an initial cohort of 105 6‐month‐old infants at 6‐month intervals with the 13C‐urea hreath test; 56 subjects were successfully studied for 2 yr or until 30 months of age. Results: Overall prevalence decreased from 71.4% to 47.9% when children were hetween 6 and 18 months of age, and we found a significant gender difference (males 63.6–55.0%, females 80.0–38.7%, p = 0.03). Of the 56 subjects, six had negative hreath tests at all 6‐month intervals, 10 were consistently positive, and 36 suhjects had one or more negative tests after a positive test. The overall prohahility of acquiring H. pylori in a given 6‐month period ranged between 0.28 and 0.38; tbe probability of clearing the infection was between 0.22 and 0.45. During the first 18 months after birth, male infants were more likely to acquire H. pylori and less likely to clear the infection than female infants. Conclusion: We conclude that H. pylori colonization in infants may be a reversible process. Copyright © 1994, Wiley Blackwell. All rights reserved