Abstract: The present invention provides genetic markers and biomarkers that are associated with anemia induced by ribavirin therapy. The genetic markers are located in the ITPA gene and elsewhere on human chromosome 20 and the biomarkers are low ITPA activity phenotypes. These markers of ribavirin-induced anemia are useful, inter alia, to identify patients who are least likely to develop anemia upon treatment with ribavirin pharmaceutical compositions and drug products, in methods of treating patients having a disease susceptible to treatment with ribavirin, and in methods for selecting the most appropriate therapy for such patients.