Interferon alfa-2a resembles other recombinant α interferons in structure and pharmacology. Studies delineating its efficacy suggest that after 6 to 12 months of therapy, sustained biochemical responses occur in 4% to 49% of patients, providing adequate doses (> or = 3 million units three times weekly) are used. Overall, a dose-response curve is evident in regard to biochemical responses, decrease in histological scores for inflammation, and virological responses (percent negative serum hepatitis C virus [HCV] RNA) after treatment. Differences in response rates between studies probably reflect patient selection and particularly the percent of patients with established cirrhosis, because these patients respond less well than those with earlier histological stages of disease. A variety of regimens of interferon alfa-2a have been used, including use of higher doses, longer periods of treatment, induction using higher doses initially, and intermittent pulse therapy. Both longer periods of therapy and induction dosing show strong but not significant trends toward improvement in response rates, which reach significance when the two strategies are combined. Outcome is not significantly improved by escalating doses during therapy when inadequate responses are seen, or by pulse therapy. The side effect profile of interferon alfa-2a is similar to that of other interferons. Long-term follow-up studies have demonstrated that 90% of patients with a sustained biochemical response at 6 months after therapy maintain the response, and 95% remain negative for HCV RNA in serum. Challenges for the future include better delineation of the advantages of induction dosing, the optimal duration of therapy, and further understanding of the frequent discordance between HCV RNA data and aminotransferase levels. This latter finding suggests that use of both parameters is necessary to evaluate fully responses to therapy.