Abstract: Aims/Background: This randomized study was performed to compare the efficacy of interferon-alpha (IFN-α) + thymosin alpha 1 (Tα1) treatment to that of IFN-α alone in light of biochemical and virological response of naive patients with chronic hepatitis C. Methods: Seventeen patients were treated with IFN α-2b (3 million units MU three times a week) + Tα1 (1 mg twice weekly); the other 17 patients received only IFN α-2b at the same dose. All patients were treated for 6 months and followed up for 12 months. Biochemical (ALT values) and virological (HCV-RNA) responses to treatment were determined. Results: Combination therapy showed significantly higher efficacy than monotherapy in achieving biochemical and virologic end-of-treatment response (p<0.05). At 12 month follow-up, the sustained biochemical response was slightly greater in patients treated with combination therapy than in those treated with monotherapy. No significant difference in response by HCV-1b subtype was observed between the two treatment groups; however, HCV-2c subtype showed a trend to responding better to IFN-α+Tα1 than to IFN-α alone. Conclusions: These data suggest that the immune modulator Tα1 may be additive or synergistic with IFN-α in normalizing end-treatment biochemical and virological responses in patients with chronic hepatitis C. Higher doses and/or more prolonged courses may improve the sustained response rates to this treatment.