To investigate correlations between the interval between blood transfusion and the start of IFN therapy, and IFN efficacy, we studied chronic hepatitis C patients with a history of blood transfusion. The subjects were 122 patients with chronic hepatitis C and a history of blood transfusion at 64 institutions. The patients were treated with high or low-dose IFN. High-dose therapy consisted of intramuscular injection of human lymphoblastoid interferon (HLBI), 6 x 10(6) IU daily for 2 weeks, then 3 times a week for 22 weeks, and low-dose interferon therapy of intramuscular injection of HLBI, 6 x 10(6) IU daily for 2 weeks, then 3 x 10(6) IU 3 times a week for 22 weeks. Normal serum ALT levels for 6 months or more after completing IFN (complete response) were found in 44/122 (36.2%) patients and HCV RNA was no longer detectable after completing IFN therapy in 19/68 (27.9%). Patients in whom the interval between blood transfusion and the start of IFN therapy was less than 20 years had significantly higher rates of HCV RNA-negative complete response than those in whom the interval was 20 years or more (p < 0.039). When chronic HCV infection is caused by blood transfusion, the efficacy of IFN depends on the duration of chronic HCV infection. Since the duration of HCV infection is a factor in predicting efficacy, early IFN therapy may be more effective.