Serum hepatitis B surface antigen (HBsAg) quantification has been suggested to reflect the concentration of covalently closed circular DNA in the liver. We aimed to investigate the HBsAg levels at different stages of chronic hepatitis B and the changes in HBsAg level during the natural progression of disease. One hundred seventeen untreated patients with chronic hepatitis B were studied with longitudinal follow-up for 99 ± 16 months. HBsAg quantification was performed at the first visit, the last visit, and three visits at each quartile during the follow-up. At the first visit, HBsAg level was higher among patients who were hepatitis B e antigen (HBeAg)-positive (N = 49) than those who were HBeAg-negative (N = 68) (4.01 ± 0.91 log IU/mL versus 2.73 ± 1.25 log IU/mL, P < 0.001). HBsAg level was persistently high at approximately 5 log IU/mL among patients in the immune tolerance phase (N = 7). The HBsAg levels among patients with HBeAg-positive active disease (N = 25) or sustained HBeAg seroconversion (N = 17) were comparable at approximately 3-4 log IU/mL. The HBsAg levels among patients who were HBeAg-negative tended to be higher among patients with active (N = 46) than those with inactive disease (N = 22). The median HBsAg levels decreased in HBeAg-negative patients with active and inactive disease by 0.041 log IU/mL/year and 0.043 log IU/mL/year, respectively. Twenty-two (17%) patients had HBsAg reduction >1 log IU/mL at the last visit; most of them showed reduced hepatitis B virus DNA, and eight had HBsAg loss. Conclusion: HBsAg remained stable in HBeAg-positive patients and tended to reduce slowly in HBeAg-negative patients. Reduction of HBsAg for >1 log IU/mL could reflect improved immune control. (HEPATOLOGY 2010)