The optimal and safer interleukin-2 (IL-2) dose for treatment of chronic hepatitis C virus (HCV) infection has been studied in 33 HCV-RNA positive patients with chronic hepatitis C. Patients were randomly allocated to receive 5 days per week during 12 weeks IL-2 doses of: 0.9 MIU (n = 10), 1.8 MIU (n = 10), or 3.6 MIU (n = 13). After 12 weeks, responder patients stopped treatment, whereas nonresponders received 12 additional weeks of IL-2 at the next higher dose: 1.8, 3.6, or 5.4 MIU. As a whole, after the first 12 weeks of IL-2 alanine aminotransferase (ALT) levels significantly decreased (P < .001) with respect to the baseline values (140 +/- 63 vs. 70 +/- 30 IU/L). At the end of treatment (24 weeks), the mean ALT level (80 +/- 50 IU/L) continued significantly lower (P < .001) than the baseline one, and 24% of patients normalized ALT levels; according to dosage, ALT normalization was: 0% for 0.9 MIU, 25% for 1.8 MIU, 5% for 3.6 MIU, and 18% for 5.4 MIU. HCV-RNA levels decreased during treatment, but in none of the patients became undetectable. All patients had a local reaction at the injection site with induration, erythema, and swelling, which was dose-related. The dose of 5.4 MIU was poorly tolerated and was reduced to 3.6 MIU in 4 of 11 patients. No changes in hematological parameters were observed. At the end of follow-up (6 months) four of eight responder patients continued with normal ALT. In conclusion, IL-2 treatment for chronic hepatitis C induced a biochemical response in 8 of 33 (24%) patients at the end of therapy while at the end of follow-up, 4 of 33 (8%) patients remained with normal ALT. The dose of 1.8 MIU is well tolerated and seems to be the most efficacious.