Helicobacter pylori is a worldwide infection. It is estimated that approximately 50% of the general population is affected, but this percentage varies considerably between countries. To investigate the prevalence of H. pylori infection, a cross-sectional epidemiological study, based on the serological determination of the IgG antibodies against H. pylori, was carried out in healthy Italian blood donors by using a commercially available kit. From March 1995 to March 1997, a total of 2598 consecutive volunteer blood donors were tested for the presence of antibodies against H. pylori. All patients answered a detailed questionnaire which collected sociodemographic characteristics, and smoking, alcohol drinking and dietary habits. Test-positive subjects with gastrointestinal symptoms underwent endoscopy, with biopsies taken for histological diagnosis. The global prevalence of H. pylori infection in our study was 1161/2598 (45%). It was directly correlated with age (67% in subjects aged > or = 50 years). The prevalence of H. pylori infection was higher in men (46.4%) than women (38.4%), and more frequent in patients with a low educational level, in the lower quintile of height and in the upper quintile of body mass index (BMI). No significant association with smoking and alcohol drinking was found. Inverse associations were found with elevated consumption of milk (chi-square for trend 5.49, P < 0.05), but not other examined food groups. Multivariate analysis selected sex, age, BMI and educational level as the variables independently related to H. pylori infection. This study confirms relatively high prevalence of H. pylori seropositivity among Italian healthy adults and points to sex, age, BMI and sociocultural class as persisting determinant features of H. pylori infection.