Abstract We studied, prospectively, seroconversion for Helicobacter pylori in adults from a developing country and investigated risk factors for the acquisition of the microorganism in this population. A group of 213 volunteers of low socioeconomic level from a district in the metropolitan area of Belo Horizonte, south-east Brazil was evaluated. Anti- H. pylori IgG antibodies were measured by ELISA using Cobas Core anti- H. pylori EIA (Roche) in serum samples collected in 1992 and in 1997. The subjects were interviewed and sociodemographic data were collected. A total of 174 (81·7%) subjects presented anti- H. pylori antibodies on the occasion of the first visit. During 56 months of follow-up, 2 of 39 seronegative adults converted to seropositive with an annual infection rate of 1·1%, and 2 of 174 seropositive subjects reverted to seronegative (0·2%/year). The prevalence of infection increased significantly with age and an inverse association was observed between prevalence of infection and educational level. In conclusion, the results of the present study demonstrate that in a developing country there is a low but continuous risk of H. pylori infection in adulthood.