The prevalence ofHelicobacter pylori infection in Zaire was determined by means of a [14C] urea breath test in 133 asymptomatic subjects, by culture and histological examination of biopsies in 324 consecutive endoscopy patients with chronic epigastric complaints, and by both the breath test and culture/histology in a subset of 92 patients. Sixty healthy Belgian students or hospital laboratory workers were also included for comparison. The prevalence ofHelicobacter pylori was significantly higher in asymptomatic Zairian subjects (77.4 %) than in the Belgians (30 %; p<10−6). Infection was also acquired much earlier in life in Africans, 66% of the children aged 5 to 9 years already being infected versus none of the Belgian subjects below the age of 20 years. In Zaire, however, the prevalence ofHelicobacter pylori in patients with gastroduodenal disorders (87.5 %) was similar to that in the group of asymptomatic subjects (77.5 %) after adjustment for age and other epidemiological parameters (gender, place of residency, education level, smoking and drinking habits) in a multivariate analysis. The high rate of acquisition ofHelicobacter pylori infection in Zaire emphasizes the need to consider the baseline prevalence ofHelicobacter pylori in a defined population when studying its association with various diseases.