Helicobacter pylori infection is widespread among Peruvian adults by the age of 30, but the age at which children become infected, the prevalence of disease, and the role of socioeconomic status in the epidemiology of infection are not known. We used the 13C-urea breath test to study the prevalence of infection in 407 Peruvian children from Lima, aged 2 months to 12 years, from families of low and high socioeconomic status. Peruvian children acquire H pylori early in life and the number of infected individuals increases rapidly with age; overall prevalence was 48%. H pylori infection was independent of sex, but was highly correlated with socioeconomic status; prevalence of infection was higher among children from low-income families than among those from high-income families (56% vs 32%, p = 0.001). Children whose homes had external water sources were three times more likely to be infected than were those whose homes had internal water sources. Among families with internal water sources, there was no difference in H pylori infection associated with income. Children from high-income families whose homes were supplied with municipal water were 12 times more likely to be infected than were those from high-income families whose water supply came from community wells. The findings show that the prevalence of H pylori infection is high among young Peruvian children and that the municipal water supply seems to be an important source of infection among Lima children from families of both low and high socioeconomic status.