During the period May 1987 to November 1989, the prevalence of hepatitis B virus (HBV) markers was determined by ELISA in serum samples of 7405 (55% male, 45% female) apparently healthy persons 3-19 years of age in Italy. Earlier studies of adults there had shown an intermediate degree of HBV endemicity (hepatitis B surface antigen carrier rate greater than 2%). Persons were selected by systematic cluster sampling in five different geographical areas of Italy. The overall prevalence of hepatitis B surface antigen (HBsAg) was 0.6%. The overall prevalence of at least one marker of HBV was 2.8%; it increased from 1.7% among children 3-5 years of age to 4.5% in teenagers 17-19 years of age (P less than 0.001). The prevalence of any HBV marker was higher in southern then in northern areas (3.5% vs. 1.8%, P less than 0.001). A significant association was found with sociodemographic features. Persons whose fathers had less than 6 years of schooling had a 2.3-fold risk (C.I. 95% = 1.5-3.4) while those belonging to a household of six or more under one roof had a 1.7-fold risk (C.I. 95% = 1.2-2.4) of previous exposure to HBV infection. These findings indicate that, today in Italy, exposure to HBV infection at a young age is very low and suggest a shift towards a low degree of endemicity following improvements in socio-economic conditions, decreased family size and increasing use of disposable syringes during recent years.(ABSTRACT TRUNCATED AT 250 WORDS)