The availability of serum samples collected in 1958 and in 1977 from healthy individuals of Milan, has provided an opportunity to compare the prevalence of anti-HAV and anti-HBs in the same area at a distance of about 20 years. The results obtained show a general drop of anti-HAV rate from 1958 to 1977. Difference in antibody prevalence was found to be significantly remarkable (35.7% versus 3.6%, P < 0.05 and 78.6% versus 26.9%, P < 0.01) in the youngest age groups examined (1-5 and 6-15 years old). Moreover the plateau of antibody acquisition was reached over 30 years of age in people selected in 1958 and over 50 years in 1977. No substantial difference was found in anti-HBs prevalence in sera collected in 1958 and in 1977 (29.2% versus 29.3%). The finding of a decline in hepatitis A infection may reflect the improvements in public health and hygienic conditions which occurred in our city in the last decades, whereas with regard to hepatitis B, the maintenance of the same incidence of infection may be due to different contributory factors.