Factors influencing the pattern of Helicobacter pylori infection among children living in adjacent urban and rural areas of northern Sardinia, Italy, were compared. The seroprevalence of H. pylori infection was 22% (625 of 2810 children) in the study population and was significantly higher among children in rural areas (37%) than in urban areas (13%) (odds ratio [OR], 3.8; 95% confidence interval [CI], 3.2-4.7; P<.005). This difference was consistent within each age group. In rural areas, children who had dogs were at greatest risk for H. pylori infection (OR, 1.8; 95% CI, 1.3-2.6; P<.05). No association was seen between H. pylori seropositivity and a history of breast-feeding. Urban children attending day care centers had a higher prevalence of infection (17%) than did those who never attended (12%) (OR, 1.5; 95% CI, 1.1-2.0; P<.05). The epidemiology of H. pylori infection is complex; even within the same geographic area, different factors influence acquisition of H. pylori infection.