Objective: To evaluate the long term effectiveness of preventive HBV infection and to monitor the incidence of hepatitis B in children to see possible impact on the program of Long An that was launched in 1986. Methods: (1) Set up a surveillance systemof hepatitis ,to evaluate the possible impact on incidence of hepatitis B. (2) To serologically evaluate the effect of the program, a stratified random sampling of 1000 subjects in 1987 birth cohorts was recruited for long term follow up at the age 1-20 years. (3) Cross-sectional seroepidemiolgical survey was carried out in the county in 1985 before the program and 20 years later. HBsAg , anti-HBs and anti-HBc were tested by RIA. Results: The average coverage of hepatitis B vaccine was 89. 1 %. At 20 years after vaccination, the seropositivity for HBsAg in population of 1-20 years has decreased from 16. 0 % to 2. 3 %, the annual effectiveness was 89. 7 %. HBV accumulated infection rate was 3. 5 %, protective rate was 96. 2 %. The incidence of acute hepatitis B was 1. 60 per 100,000 in population aged 1 -20 years , it decreased by 91.3 % as compared with the incidence of 18.38 per 100,000 in same age group in 1985 -1987. Conclusion: Mass hepatitis B vaccination program in Long An County has proved to be effective in control of HBV chronic infection and incidence of acute hepatitis B. Background and Aims: Although the evolution of viral quasispecies may be related to the pathological condition of disease, little is known about this in hepatitis B virus (HBV), especially during HBeAg seroconversion. Methods: Nucleotide sequences of HBV precore/core genes from 5 time points were analyzed in four cohorts of chronic hepatitis B, interferon-induced seroconverters (IS, N=9), interferon non-responders (IN, N=9), spontaneous seroconverters (SS, N=9) and non-seroconverters (SN, N=9), followed during 60 months on average. Only patients with genotype C were used. Viral diversity was then estimated after nucleotide genetic distance was assessed and phylogenetic trees were constructed. Results: Analysis of 1800 nucleotide sequences showed that the nucleotide genetic distance of seroconverters (IS and SS; 9x10 -3 substitutions/site and 8.5x10 -3 subsititutions/site, respectively) was similar to that of non-seroconverters (IN and SN; both 7x10 -3 substitutions/site) before seroconversion. Compared to that of nonseroconverters (IN and SN; substitutions/site and 7.5x10 -3 substitutions/site, respectively) the viral diversity of seroconverters (IS and SS; 13x10 -3 substitutions/site and 12x10 -3 substitutions/site, respectively) was significantly higher after seroconversion (p<0.05) and it was higher after seroconversion in seroconverters compared with that berore seroconversion (p<0.05) while it almost didn't change in non-seroconverters irrespective seroconversion. Phylogenetic trees also showed that complex trees appeared in secoconverters and relatively simple in nonseroconverters. Conclusions: The distinctly higher viral diversity after seroconversion in HBeAg seroconverters could be related to increased HBV-specific T-cell responses and escape mutant which arise from stronger selective pressure caused by host immune activity.