Some patients with chronic hepatitis C become HCV-RNA seronegative during interferon (IFN) therapy. However, about one-half of these patients show a relapse, evident by high serum alanine aminotransferase (ALT) level. In some patients with biochemical relapse, the serum HCV-RNA level becomes low immediately after the ALT relapse. Here, we assessed the changes in serum HCV-RNA level in patients with ALT relapse after IFN therapy, and evaluated the efficacy of a second course of IFN, started at the recovery stage after ALT relapse. Two hundred and seventy-seven patients who showed HCV-RNA seronegativity by reverse transcription nested-polymerase chain reaction (RT nested-PCR) and normalization of ALT during the initial IFN therapy, and had positive HCV-RNA with ALT relapse (> 100 IU/l) within 3 months after completion of the initial IFN course were enrolled in this retrospective study. Two hundred and sixty patients were followed-up without further IFN retreatment after the ALT relapse (group 1), and 17 patients received another 6-month course of IFN after the ALT relapse (group 2). The median level of serum HCV-RNA, determined with a branched DNA probe assay (version 1; Chiron-Dai-ichi Kagaku Tokyo, Japan), in group 1 was 3.1 Meq/ml before IFN therapy, 1.3 Meq/ml at the time point of the ALT peak after the completion of IFN therapy, and 0.7 and 2.6 Meq/ml at 2-4 and 6-8 weeks after the ALT peak, respectively. The serum HCV-RNA level at 2-4 weeks after the ALT peak was lower than that before IFN therapy. The eradication rate of HCV-RNA (complete response; CR) in group 2 (47.1%; 8/17) was significantly higher than that in group 1 (1.5%; 4/260; P < 0.001). In conclusion, our data suggested that: (1) patients who showed biochemical relapse after initial IFN therapy had a significantly lower serum HCV-RNA level at recovery after ALT relapse compared with that before initial IFN therapy. (2) A high response rate was noted after a second course of IFN administered at the recovery stage of the ALT relapse, compared with patients without IFN retreatment.