Summary. The treatment paradigm for hepatitis C virus (HCV) infection is at a critical point in its evolution. The addition of a protease inhibitor to peginterferon plus ribavirin has become the new standard-of-care treatment for most patients. Data from clinical trials of new antivirals have been difficult to interpret and compare, partly because of heterogeneity in trial design, and partly because of inconsistencies in terminology used to define viral responses and the populations evaluated. Present definitions of viral responses for treatment with peginterferon and ribavirin are insufficient for novel treatment paradigms. Further, categorization of prior patient treatment experience in clinical trials, particularly of nonresponders to prior therapy, is inconsistent. Existing terms and definitions must be updated, standardized and/or redefined for easier interpretation of data and effective communication among clinicians. A panel of experts in HCV infection treatment met on 3 December 2009. Goals of the panel were to evaluate terms and definitions used traditionally in treatment with peginterferon and ribavirin, to refine and clarify definitions of existing terms that have varying meanings and to propose new terms and definitions appropriate for novel treatment paradigms emerging with development of new agents. A number of recommendations were accepted unanimously by the panel. Adoption of these terms would improve communication among investigators, enhance comparability among clinical trials, facilitate development of therapeutic guidelines and provide a standardized terminology for use in clinical practice.