The importance of serum hepatitis B surface antigen (HBsAg) level as a surrogate marker for viral load and a predictor of treatment response remains unclear. The aim of this study was to investigate whether serum HBsAg correlates with serum hepatitis B virus (HBV) DNA during peginterferon (PEG-IFN) α-2a treatment (with or without thymosin α-1) in hepatitis B e antigen (HBeAg)-positive chronic hepatitis B patients and whether it can predict treatment response. Sera from 37 HBeAg-positive chronic hepatitis B patients receiving 48-weeks PEG-IFN α-2a with (n = 20) or without (n = 17) an initial 12-weeks thymosin α-1 were obtained at baseline and at weeks 12, 24, 36, 48 (end of treatment), 56, 72, 84, and 96 (end of follow-up). Taqman HBV DNA tests (Roche) and Architect HBsAg QT (Abbott) were performed. There was a moderate correlation between the HBsAg and HBV DNA levels (r = 0.452, P < 0.001). Median HBsAg levels at baseline and at week 96 were 6,218 IU/ml and 4,038 IU/ml, respectively. The mean HBV DNA and alanine aminotransferase (ALT) levels were 7.48 log10 IU/ml and 173 IU/L at baseline and 5.37 log10 IU/ml and 102 IU/L at week 96, respectively. A decrease to <60% of baseline levels of HBsAg at week 12 was identified as an independent predictive factor for HBeAg seroconversion (OR = 45.7, P < 0.05) at week 96. Serum HBsAg levels may be helpful for predicting the response to PEG-IFN therapy in HBeAg-positive chronic hepatitis B patients. J. Med. Virol. 83:88–94, 2011. © 2010 Wiley-Liss, Inc.