The efficacy and safety of peginterferon-alpha-2a (40 kD) (PEG-IFNalpha-2a), 450 microg once weekly, versus IFNalpha-2a, 9 MIU once daily, for 12 months, was evaluated in a Phase II study in IFN-naïve patients with chronic-phase, Philadelphia-chromosome-positive CML. At the end of the treatment, complete hematological response was observed in 66.2% (47/71) and 45.2% (33/73) of the PEG-IFNalpha-2a group and IFNalpha-2a groups, respectively (p = 0.009), and major cytogenetic response occurred in 35.2% and 17.8%, respectively (p = 0.016). PEG-IFNalpha-2a was at least as effective as IFNalpha-2a overall, including progression-free survival at the end of treatment, and overall survival after 30 months of follow-up. Adverse events necessitated fewer withdrawals but more dose adjustments in the PEG-IFNalpha-2a group compared with the IFNalpha-2a group (11%versus 23%, and 84.5%versus 65.8%, respectively). In conclusion, PEG-IFNalpha-2a (40 kD), 450 microg once weekly, compared with IFNalpha-2a, 9 MIU once daily, resulted in higher rates of hematologic and cytogenetic response and greater overall survival.