BACKGROUND: Helicobacter pylori (H. pylori) infection is ubiquitous in sub-Saharan Africa, but paradoxically gastric cancer is rare. METHODS: Sera collected during a household-based survey in rural Tanzania in 1985 were tested for anti-H. pylori IgG and IgG subclass antibodies by enzyme immunoassay. Odds ratios (OR) and confidence intervals (CI) of association of seropositivity with demographic variables were computed by logistic regression models. RESULTS: Of 788 participants, 513 were aged < or = 17 years. H. pylori seropositivity increased from 76% at 0-4 years to 99% by > or = 18 years of age. Seropositivity was associated with age (OR 11.5, 95% CI 4.2-31.4 for 10-17 vs. 0-4 years), higher birth-order (11.1; 3.6-34.1 for > or = 3rd vs. 1st born), and having a seropositive next-older sibling (2.7; 0.9-8.3). Median values of IgG subclass were 7.2 for IgG1 and 2.0 for IgG2. The median IgG1/IgG2 ratio was 3.1 (IQR: 1.7-5.6), consistent with a Th2-dominant immune profile. Th2-dominant response was more frequent in children than adults (OR 2.4, 95% CI 1.3-4.4). CONCLUSION: H. pylori seropositivity was highly prevalent in Tanzania and the immunological response was Th2-dominant. Th2-dominant immune response, possibly caused by concurrent bacterial or parasitic infections, could explain, in part, the lower risk of H. pylori-associated gastric cancer in Africa.