Interferon lambda 1 (IFN-k1) is a type III IFN that produces intracellular responses similar to those of IFN-a but in fewer cell types because of differences in the receptor distribution pattern, and this could potentially result in an improved safety profile. This was an open-label three-part study of patients with chronic hepatitis C virus (HCV) genotype 1 infection. Part 1 evaluated single-agent pegylated interferon lambda (PEG-IFN-k) at 1.5 or 3.0 lg/kg administered every 2 weeks or weekly for 4 weeks in patients who had relapsed after previous IFN-a-based treatment. Part 2 evaluated weekly doses of PEG-IFN-k ranging from 0.5 to 2.25 lg/kg in combination with ribavirin (RBV) for 4 weeks in treatment-relapse patients. Part 3 evaluated weekly PEG-IFN-k at 1.5 lg/kg in combination with RBV for 4 weeks in treatment-naive patients. Fifty-six patients were enrolled: 24 patients in part 1, 25 patients in part 2, and 7 patients in part 3. Antiviral activity was observed at all PEG-IFN-k dose levels (from 0.5 to 3.0 lg/kg). Two of seven treatmentnaive patients (29%) achieved rapid virological response. Treatment was well tolerated with minimal flu-like symptoms and no significant hematologic changes other than RBV-associated decreases in hemoglobin. The most common adverse events were fatigue (29%), nausea (12%), and myalgia (11%). Six patients experienced increases in aminotransferases that met protocol-defined criteria for dose-limiting toxicity (DLT) or temporarily holding therapy with PEG-IFN-k. Most DLT occurred in patients with high PEG-IFN-k exposure. Conclusion: Weekly PEG-IFN-k with or without daily RBV for 4 weeks is well tolerated with minimal adverse events and hematologic effects and is associated with clear antiviral activity across a broad range of doses in patients with chronic HCV.