Background and aims: Flares are a well known phenomenon during antiviral treatment for chronic hepatitis B. Little is known about the effect of flares on response. We investigated the timing and characteristics of flares, in relation to treatment response (hepatitis B e antigen loss). Patients: A total of 266 patients, participating in a global randomised controlled study, were assigned to 52wee ks of 100 μg pegylated (Peg)-interferon a-2b weekly, combined with either dai ly lamivudine 100 mg or placebo. Results: Sixty seven patients (25%) exhibited 75 flares, with 38 (51%) flares in the combination therapy and 37 (49%) in the monotherapy groups. Overall, 30%of patients with and 38%of patients without a flare responded to therapy (p = 0.25). In 24 patients (36%) the flare was foll owed by a decrease in hepatitis B virus (HBV) DNA (host induced flare). In 25 (3 8%) patients the flare was preceded by an increase in HBV DNA (virus induced fl are). In 17 (26%) patients the flare did not meet one of these criteria (indete rminate flare). Of patients with host induced flare, 58%responded whereas only 20%of patientswith virus induced flares responded (p = 0.008). Hepatitis B surf ace antigen loss (n = 8) was exclusively seen in patients experiencing a host in duced flare. Multivariate logistic analysis showed that host induced flares was an independent predictor of response (p = 0.043). Conclusion: Flares are not mor e common in responders than in non-responders to Peginterferon α-2b therapy. Virus induced flares, which occur after an increase in HBV DNA level, and most p robably are indicative for increased expression of viral antigens, did not lead to treatment response. In contrast, host induced flares which were followed by a HBV DNA decrease were highly associated with treatment response. Background and aims: Flares are a well known phenomenon during antiviral treatment for chronic hepatitis B. Little is known about the effect of flares on response. We investigated the timing and characteristics of flares, in relation to treatment response (hepatitis B e antigen loss) Patients: A total of 266 patients, participating in a global randomized controlled study, were assigned to 52 persons ks of 100 μg pegylated (Peg) -interferon a-2b weekly, combined with either lycopene 100 mg or placebo. Results: Sixty Overall, 30% of patients with and 38% of patients without a flare responded to therapy (25%) exhibited 75 flares, with 38 (51%) flares in the combination therapy and 37 In 25 patients (36%) the flare was foll owed by a decrease in hepatitis B virus (HBV) DNA (host induced flare). In 25 (3 8%) patients the flare was preceded by an increase in HBV DNA (virus induced fl are). In 17 (26%) patients the flare did not m Of patients with host induced flare, 58% had only only 20% of patients with virus induced flares responded (p = 0.008). Hepatitis B surf ace antigen loss (n = 8) was exclusively Multivariate logistic analysis showed that host induced flares was an independent predictor of response (p = 0.043). Conclusion: Flares are not mor e common in responders than in non-responders to Peginterferon α-2b Virus induced flares, which occur after an increase in HBV DNA level, and most p robably are indicative for increased expression of viral antigens, did not lead to treatment response. were highly associated with treatment response.