Backgrounds/Aims: We examined the serologic markers of hepatitis B virus (HBV) infection inclu ding anti-HBc (IgG) among Korean adults in the late 1990s to analyze the changing trends of HBV infection. Methods: For 70,347 adults who visited Asan Medical Center for general health screening from July, 1995 to December, 1997, HBsAg, anti-HBs and anti-HBc were tested by radioimmunoas say and serum alanine aminotransferase (ALT) was examined. Results: The overall prevalence of HBsAg was 5.7% and was increased with age (2.5% in teens, 5.4% in twenties, 6.8% in thirties) The overall prevalence of anti-HBc was 61.6%, which was significantly different below and above thirties. The proportion of persons who were negative for all HBV markers was 12.6% and this rate was similar in all age group. The prevalence of anti-HBc among persons who were negative in HBsAg and anti-HBs increased markedly with age. The percentage of persons with elevated ALT (≥ 2x upper normal limit) was higher in HBsAg-positive subjects than in HBsAg-negative subjects (7.4% vs 2.4%). The rare combination of HBV markers (+/-/-) which were positive in HBsAg and negative in anti-HBs and anti-HBC was found in 0.06% of entire subjects. Conclusions: The prevalence of HBsAg was decreasing and the percentage of subjects who were anti-HBc-negative was high in younger age group ( 30 years). Although the aquisition of anti-HBs was increasing through HBV vaccination, a significant proportion (>10%) without any HBV markers was still susceptible to HBV infection. (Kor J Gastroenterol 1999;33:642 - 652)