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Helicobacter pylori is one of the most common bacterial infections worldwide. To evaluate the prevalence of this infection in Gipuzkoa (Basque Country, Spain) we studied the presence of antibodies against Helicobacter pylori (HPAb) using a second-generation EIA (Cobas Core). The study was performed on two groups of subjects: a middle-class group, 2-78 years-old (n = 1335) and a group of slum dwellers, 2-15 years-old (n = 89). In the middle-class group the prevalence of HPAb in children under 6 was 3.1% (3/96); the prevalence was significantly greater in older compared to younger age groups, reaching 84.3% (102/121) in adults 50-59 years. The geometric mean of the titer in seropositive subjects was also greater in older age groups. By logistic regression analysis the prevalence of HPAb was associated with age, educational level and geographic origin but not with sex, smoking, alcohol consumption, or use of nonsteroid anti-inflammatory drugs. The prevalence of HPAb was much higher in the slum-dwelling group 2-15 years-old (55.5% of children 2-5 years-old). The results indicate that H. pylori infection was more common in adult people from our geographic region than in those from other developed countries and show that socioeconomically deprived children constitute at present a group at high risk of acquiring infection in our region.