Recombinant interferon alpha-2b (rIFN-alpha2b) is an effective therapy for chronic-phase chronic myelogenous leukemia (CML). Polyethylene glycol-modified rIFN-alpha2b is a novel formulation with a serum half-life ( approximately 40 h) compatible with once-weekly dosing. This open-label, noninferiority trial randomized 344 newly diagnosed CML patients: 171 received subcutaneous pegylated rIFN-alpha2b (6 microg/kg/week); 173 received rIFN-alpha2b (5 million International Units/m2/day). Primary efficacy end point was the 12-month major cytogenetic response (MCR) rate (<35% Philadelphia chromosome-positive cells). Modified efficacy analysis included all MCRs >12 months, except for patients discontinuing treatment after 6 months and achieving an MCR on other salvage therapy. The MCR rates were 23% for pegylated rIFN-alpha2b vs 28% for rIFN-alpha2b in the primary efficacy analysis and 26 vs 28% in the prospectively modified efficacy analysis. However, a significant imbalance in baseline hematocrit (HCT), a significant predictor of cytogenetic response (P=0.0001), was discovered: 51 (30%) patients treated with pegylated rIFN-alpha2b had low HCT (<33%) vs 33 (19%) rIFN-alpha2b-treated patients. Among patients with HCT >33%, the MCR rate was 33 vs 31%. The adverse event profile of weekly pegylated rIFN-alpha2b was comparable to daily rIFN-alpha2b. Once-weekly pegylated rIFN-alpha2b is an active agent for the treatment of newly diagnosed CML with an efficacy and safety profile similar to daily rIFN-alpha2b, although statistical noninferiority was not demonstrated.