Summary. The efficacy and tolerability of 12-month treatment with titrated doses of recombinant interferon-α2a (IFN-α2a) in chronic hepatitis C were studied in 67 consecutively recruited patients randomly assigned either to a starting dose of IFN-α2a 6 MU, subsequently adjusted to the serum alanine aminotransferase (ALT) response (n= 35), or to no therapy (n= 32; controls). End-of-treatment ALT levels were normal and hepatitis C virus (HCV) RNA was negative by nested polymerase chain reaction (PCR) in 17 (49%) treated patients compared to none of the controls (P < 0.001). During the 12 months after stopping treatment the number of patients who remained in remission was eight (23%) and one respectively (4%) (P= 0.031). Follow-up liver biopsy showed reduced hepatic inflammation in 80% of treated patients and in 29% of controls (P < 0.001). The eight sustained responders and 2 7 non-responders or relapsers received similar mean total doses of IFN (565 MU vs 545 MU) and had a similar incidence of anti-IFN neutralizing anti-bodys (13% vs 19%). Absence of cirrhosis was the only independent pretreatment parameter that predicted a sustained response. In conclusion, a mean cumulative dose of IFN 549 MU, titrated over 12 months, was well tolerated, and resulted in the long-term clearance of HCV RNA and normal ALT levels in 23% of patients.