Background/Aims: Hepatitis B and C viruses (HBV and HCV) are two clinically distinct but related diseases. Pooled data from five studies of peginterferon alpha-2a in patients with chronic HCV infection (CHC) were compared with two studies of the drug in patients with chronic HBV infection (CHB). Method: The HBV studies included both hepatitis B e antigen (HBeAg)-positive (n=271) and HBeAg-negative (n=177) patients; 791 patients took part in the HCV trials. In all studies, patients were treated with 180 mu g peginterferon alpha-2a monotherapy once weekly for 48 weeks. The number of adverse events (AEs), discontinuations and dose modifications were documented. Health-related quality of life (HRQL) was assessed using the Short-Form 36 questionnaire. Safety was assessed throughout the treatment period. A 24-week treatment-free follow-up period was also included. Results: Differences (HBV vs HCV) were observed in the incidence of AEs (88-89 vs 96-100%), serious AEs (4-5 vs 7-16%) and treatment withdrawals (6-8 vs 17-33%). The frequency of depression-related events was lower in CHB patients (4 vs 22%, P < 0.001), as was the impact of treatment on HRQL. Conclusions: The safety and tolerability of peginterferon alpha-2a in patients with CHB compares favourably with that observed in CHC patients, with a lower incidence of common interferon-related AEs and a significantly lower incidence of depression.