This study assessed the role of parental infection status in the transmission of Helicobacter pylori infection in a large population-based sample of preschool-aged children. The subjects, who lived in Ulm, Germany, and in two nearby communities, were screened for school fitness between January and July 1997. Their H. pylori infection status was determined by 13C-urea breath test. Of 1522 eligible children, 1221 (80.2%) participated in the study. Crude prevalence of H. pylori infection in children was 11.3% (95% confidence interval [CI], 9.5-13.3) and 36.4% in their parents (95% CI, 33.5-39.4). The crude odds ratio (OR) for H. pylori infection of children whose mothers were infected was 16.5 (95% CI, 8.9-30.8) and 7.9 after adjustment for potential confounders (95% CI, 4.0-15.7). The crude OR if the child's father was infected was 7.8 (95% CI, 2. 5-24.2) and 3.8 after adjustment for potential confounders (except maternal infection) (95% CI, 0.8-19.1). The results suggest that infected parents, especially infected mothers, may have a key role in transmission of H. pylori within families.