AIM: To determine seroprevalence of Helicobacter pylori(H pylori) in the Dunedin Multidisciplinary Health and Development Study (DMHDS) at age 26 in order toinvestigate seroconversion and seroreversion from age 11 to 26 and the association of seropositivity with risk factors for H pylori infection. METHODS: Participants in the DMHDS at age 26 and retrospectively at age 21 were tested for H pylori antibodies using two commercially available ELISA kits. Gender, socioeconomic status (SES), smoking, educational attainment and employment at age 26 were tested for association with H pylori seropositivity.RESULTS: At ages 21 and 26, seroprevalence of H pylori using one or other kit was 4.2% (n = 795) and 6.3% (n = 871) respectively. Seroreversion rate was lower than serocon version rate (0.11% vs0.53% per person-year) in contrast to the period from age 11 to 21 when seroreversion rate exceeded seroconversion rate (0.35% vs 0.11% per person-year). Serology in those tested at ages 11, 21, and 26 remained unchanged in 93.6% of the sample. Seroprevalence at age 26 was lower among those with a secondary school qualification (P = 0.042) but was not associated with gender, SES, smoking or employment status. CONCLUSION: H pylori seroprevalence in a New Zealandbirth cohort remains low between ages 11 and 26. H pylori infection remains stable from childhood to adulthood although seroreversion seems to be more common in the adolescent years than in young adults.