A large-scale seroepidemiologic study of 20816 (35.6% of total) elementary and junior high school children (grades 1-9) in Keelung, Taiwan was conducted in April-May 1988. The study was to investigate the current status of hepatitis B virus (HBV) infection and to plan the extension of the present HBV prevention program to other age groups. Two HBV markers, hepatitis B surface antigen (HBsAg) and hepatitis B surface antibody (anti-HBs), were determined by enzyme immunoassay. The overall seropositivity rates of HBsAg and anti-HBs were 19.0% and 25.6%, respectively. Only 0.4% of the children studied were concurrently positive for both HBsAg and anti-HBs and 55.8% of them were negative for these two markers. It is noteworthy that the seropositivity rate of HBsAg increased with increasing age, from 15.4% in grade 1 (ages 6) to 23.7% in grade 9 (ages 15). This rate of HBsAg was not higher than the 1986 findings (14.0-25.5%) screened from children (4-13 years old) in Keelung, by comparing each age cohort strata, even though the age of study population was two years older. These data indicate that the current hepatitis control strategies have resulted in the reduction of the hepatitis B carrier rate among children from more than 20% to 15% at age 6 (grade 1) in Keelung, but the reduction is still higher than in Taipei children (10%). The seropositivity rate of anti-HBs was also increased with increasing age, from 13.5% (grade 1) to 44.1% (grade 9) and was similar to previous findings in Taiwan.(ABSTRACT TRUNCATED AT 250 WORDS)