Risk factors for infection with Helicobacter pylori (HP) were investigated in a cohort study of 121 seronegative children in Ethiopia aged 2-4 y, who had previously participated in a case-control study. Blood samples were drawn at inclusion in the cohort study and again after 12 and 30 months. At 12 months the parents were also interviewed about putative risk factors for infection, using a structured questionnaire. Analyses were made by comparing risk factors in seropositive and seronegative children. The seroconversion rate during the first year was 31% (27/87) and during the following 18 months 34% (17/50; corresponding to an annual incidence of 24%). After a period of 30 months, 58% (14/24) of the children who were 24-29 months old at inclusion in the study had seroconverted, compared with 40% (4/10) of those who were 30-35 months old, 73% (22/30) of those who were 36-41 months old and 31% (4/13) of those who were 42 months old or more. These results indicate that peak age for HP infection is below 6 y in this cohort in Ethiopia, and might also reflect a pattern of repeated seroconversion and sero-reversion in early childhood. Independent predictors of HP-seroconversion were the variables 'drinking-water', comparing water from a well with water from rivers or pipes (RR = 1.46, 95% CI 1.0-2.15) and 'antibiotic treatment' (RR = 1.84, 95% CI 1.16-2.92).