The usefulness of fully automated chemiluminescent microparticle immunoassay (Architect HBsAg QT) for monitoring serum levels of hepatitis B virus (HBV) during antiviral therapy remains unclear. Using this assay, hepatitis B surface antigen (HBsAg) was measured in 20 patients with chronic hepatitis B before and during lamivudine treatment. At the start of therapy, 12 patients had detectable hepatitis B e antigen (HBeAg) and 8 did not. The median serum HBV DNA level and HBsAg concentration (25th–75th centile) were 7.2 (6.1–7.8) log genome equivalents/ml and 3,932 (1,585–12,330) IU/ml, respectively. The HBsAg concentration was significantly higher in HBeAg positive than in HBeAg negative patients (P = 0.031). There was a significant correlation between the HBsAg concentration and HBV DNA level (r = 0.490, P = 0.027). The HBsAg concentration negatively correlated with patient age (r = −0.395, P = 0.085). After the start of lamivudine therapy, HBV DNA levels fell rapidly in all patients. Serum HBsAg concentrations also fell in most patients, but to a lesser extent. When drug-resistant variants emerged, serum HBsAg usually increased before biochemical breakthrough. Although HBV DNA was elevated persistently after the emergence of drug-resistant variants, the increase in HBsAg was transient. In some patients, the increase in HBsAg preceded the increase in HBV DNA. Monitoring of serum HBsAg concentrations with the use of Architect HBsAg QT, in addition to measurement of HBV DNA levels, is helpful for evaluating the response to lamivudine treatment and for the early detection of drug-resistant strains. J. Med. Virol. 75:235–239, 2005. © 2004 Wiley-Liss, Inc.