Hepatitis B e antigen (HBeAg) was quantified before, during and after interferon-α administration in a trial of 162 chronic hepatitis B patients treated for 16 or 32 weeks. In 139 of these patients we examined the prognostic value of the pretreatment level of HBeAg and the reduction in HBeAg level at weeks 4 and 8 for response at week 16. Multivariate analysis showed that the HBeAg pretreatment level is a highly significant predictor of response (judged as HBeAg and hepatitis B virus [HBV] DNA negativity), followed by a decrease in HBeAg from the start of therapy to week 8. During the first 8 weeks of therapy, a decrease in HBeAg of less than 40%, as observed in 30% of the patients, consistently resulted in non-response. After 16 weeks of treatment, non-responding patients were randomly assigned to receive no further treatment (n=57) or prolonged treatment for another 16 weeks (n=61). In both groups, changes in the HBeAg level from the start of (the first) therapy to week 8, but not the pretreatment HBeAg level itself, were significantly related to the response at week 52 (the end of follow-up). Changes in the HBV DNA level had no additional predictive value for response at weeks 16 or 52. Therefore, instead of sequential HBV DNA assessment, we recommend monthly monitoring of HBeAg during IFN-α therapy.