It has been understood for some time that the treatment outcome of hepatitis C virus (HCV) infection is influenced by host genetic factors. Three independent genome-wide association studies have recently identified that a genetic variation in the IL28B gene [interferon-?3 (IFN-?3)] determines the outcome of IFN-?-based therapy in patients with genotype 1 chronic hepatitis C infection. This genetic polymorphism is also strongly associated with a higher likelihood of spontaneous clearance following acute hepatitis C infection. These results confirm the importance of specific host genetic markers in predicting outcome and treatment response. They also provide the framework and potential for a clinically relevant and meaningful pharmacogenomic approach to personalizing anti-HCV treatment.