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In 1990-1991, the prevalence of antibodies to hepatitis A virus infection (anti-HAV) was assayed by the ELISA method among 1528 apparently healthy subjects, 20-85 years old in the Republic of San Marino. Subjects were selected from the list of residents by a random stratified sampling procedure with a proportional allocation by age, sex and district of residence. The overall anti-HAV prevalence was 64.7%; it increased from 28.6% in subjects 20-30 years old to 97% in those > 60 years (p < 0.01). No gender difference was observed. At the multivariate analysis age > 40 years (OR: 39.5; 95% CI: 12.4-126) and lowest level of schooling (OR: 1.8; 95% CI: 1.1-2.9), which is a good indirect indicator of socio-economic status, resulted both independent predictors of anti-HAV seropositivity. These findings reflect the improved sanitation standards in this area and indicate that the proportion of non-immune adults is increasing with a higher risk of symptomatic infection in the near future.