Rates of Helicobacter pylori infection are traditionally higher in developing countries than in developed countries, but the specific reasons for these differences are not fully clear. While chronic diseases resulting from H. pylori are generally of adult onset, chronic infection usually begins in childhood. In this cross-sectional study (1998-2000), the authors compared prevalences of H. pylori infection among children under age 6 years on both sides of the Rio Grande. Participants included 264 children of women from low-income families who were receiving services at health clinics in Juarez, Mexico, or El Paso, Texas, from April 1998 through October 2000. Data were collected through personal interviews and serologic testing for H. pylori antibodies. The crude odds ratio for H. pylori prevalence among Mexican children as compared with US children was 3.94 (95% confidence interval: 1.72, 9.06). After adjustment for covariates, the odds ratio decreased to 1.70 (95% confidence interval: 0.64, 4.52). The adjustments that produced the greatest reduction in the odds ratio for location were those for household crowding and maternal education. This study identified specific factors that may explain geographic variation in H. pylori prevalence among children.