Hepatitis B virus (HBV) infections are endemic in Korea. The aims of this study were to determine the prevalence of HBsAg positivity in Korea and to evaluate the changes in intrafamilial transmission after introduction of HBV vaccination in 1983. This study was based on the 2001 Korea National Health and Nutrition Examination Survey. A total of 2512 study subjects, aged 10-29 years, were selected from across Korea using a stratified multi-stage probability sampling design. To identify the changes in intrafamilial transmission after the introduction of the HBV vaccination programme, 1850 subjects with parental serological markers were selected. These subjects were then grouped into two birth cohorts (cohort 1: born before 1983; cohort 2: born after 1983). Appropriate sampling weights were used for all analyses. The weighted age-specific prevalence of HBsAg was 4.9% in participants in their 20s and 1.9% in adolescents; the combined weighted prevalence was 3.2%. Of subjects with HBsAg positivity in either parent, 17.5% were HBsAg-seropositive. Of subjects with two HBsAg-negative parents, 1.5% were HBsAg-seropositive. The HBsAg positivity rate of offspring with HBsAg-positive mothers was higher than those with HBsAg-positive fathers (27.3% vs. 4.8%, P<0.001). The weighted HBsAg positivity rate of offspring with HBsAg-negative mothers was 2.3% for cohort 1 and 0.4% for cohort 2 (P<0.01), and for those offspring with HBsAg-positive mothers it was also significantly decreased compared to cohorts 1 and 2 (40.2% vs. 16.4%, P<0.01). However, the weighted HBsAg positivity rate of offspring with HBsAg-positive mothers was still high. Our results showed that introduction of HBV vaccination has resulted in a decline in the overall HBsAg positivity rate and a reduction in intrafamilial transmission in Korea, but further preventive measures for maternal intrafamilial transmission are needed.