Taiwan is an endemic area of hepatitis B virus (HBV) infection. A nationwide mass vaccination program to prevent HBV infection was started in 1985. Perinatal and horizontal transmission of HBV decreased substantially after the launching of this program. However, the influence of this program on children born before 1985 has not been studied. From 1991 to 1999, annual surveys of hepatitis B virus surface antigen (HBsAg) and antibody (anti-HBs) were carried out in freshmen at two high schools in Hualien, Taiwan. The average age was 16 years old. Although these students were born 2–10 years after the start of the national HBV vaccination program, there is a significant trend of decreasing HBsAg carrier rate (from 21.0% to 10.5% in males and 14.3% to 4.7% in females) and increasing anti-HBs rate (from 56.6% to 67.8% in males and 70.3% to 75.9% in females) over the 9 years. With yearly comparison, the carrier rate of HBsAg started to show significant decrease since 1994, while the anti-HBs began to rise significantly after 1996, especially in male students. The HBsAg carrier rate in male students was significantly higher, while the anti-HBs rate was significantly lower, than that in female students in most of the years. It is concluded that the effect of HBV vaccination also reduced horizontal transmission of HBV to children born up to 7 years before the start of the program. J. Med. Virol. 67:472–476, 2002. © 2002 Wiley-Liss, Inc.