Summary. Serum hepatitis B surface antigen (HBsAg) levels reflect intrahepatic hepatitis B virus (HBV) covalently closed circular DNA and may be a valuable addition to HBV DNA in the management of patients with chronic hepatitis B (CHB). Among HBeAg-negative CHB patients with low HBV DNA levels, HBsAg quantification may help distinguish those with active CHB from true inactive carriers with a very favourable prognosis, thus limiting the need for long-term intensive monitoring of ALT and HBV DNA levels. In patients treated with peginterferon (PEG-IFN), achievement of a decline in HBsAg during therapy appears to be an important marker for treatment outcome, and several groups have proposed stopping rules based on HBsAg thresholds. A recently described stopping rule incorporating a combination of HBsAg and HBV DNA levels can accurately identify HBeAg-negative patients, especially those with HBV genotype D, not responding to PEG-IFN. Current applications of HBsAg levels in the monitoring of patients treated with nucleo(s)tide analogues are still being evaluated. First data from these studies show that HBsAg decline, and thus subsequent clearance, is confined to those with an active immune response to HBV, such as HBeAg-positive patients with elevated ALT, or those who achieve HBeAg clearance.