OBJECTIVES To estimate patient preferences for attributes of hepatitis C virus (HCV) treatments and the effect of product attributes on stated medication adherence. METHODS HCV patients,18 years and older completed an online survey instrument that included 9 conjoint choice-format trade-off tasks. Subjects chose among pairs of hypothetical medication alternatives, each defined by chance that the medicine will eliminate the virus completely (EFFICACY), injection frequency, duration of flu-like symptoms after each injection, injection device (DEVICE), average number of days of work missed each week (LOST WORK DAYS), probability of reversible hair thinning while on treatment (HAIR THINNING), and probability of developing clinical depression while on treatment (DEPRESSION). Subjects also answered 3 rating questions assessing how often people with HCV would miss or skip doses of medications with different characteristics. We used mixed-logit methods to estimate mean relative importance weights for each attribute. We used a Heckman two-stage model to estimate first the effect of subject characteristics the effect of medication attributes on non-adherence. RESULTS A total of 143 subjects completed the survey. In the model, number of flu days (FLU DAYS) was specified as injection frequency multiplied by the duration of flu-like symptoms after every injection. EFFICACY was the most important attribute. The remaining attributes were ranked in order of importance as follows: DEPRESSION, FLU DAYS, LOST WORK DAYS, HAIR THINNING, and DEVICE. Subjects with prior experience were less likely to be non-adherent and increases in the number of flu days increased the likelihood of non-adherence. CONCLUSIONS The results of this study demonstrate that efficacy is the most important medication attribute to patients but medication side effects like the number of flu days affect stated medication adherence. Reducing the number of flu days by reducing the frequency of injections or the duration of flu-like symptoms after each injection may increase medication adherence.