We examined the levels of serum N-terminal peptide of type III procollagen (P-III-NP) and the 7S domain of type IV collagen (IV-7S) as fibrogenesis markers in patients with chronic hepatitis C to clarify whether high-dose interferon-alpha (IFN-alpha) therapy has a suppressive effect on hepatic fibrogenesis for a long period (over 5 years) after the cessation of IFN therapy. Eighty patients with CHC were given 10 million units of IFN-alpha2b daily for 14 days followed by three times per week for a total of 24 weeks. Patients were divided into the following three groups according to the highest serum alanine aminotransferase levels during 1 year observation after the end of IFN therapy: complete responders (CR), partial responders (PR), and nonresponders (NR). We measured serum fibrogenesis markers before and at the end of IFN therapy, and again 1 year and more than 5 years after the end of IFN therapy. Liver biopsies were performed before IFN therapy in all patients and again over long-term observation in 10 patients (PR; 5 and NR; 5). Serum P-III-NP levels significantly decreased after IFN therapy in all three groups of patients, and further decreased in CR and PR over long-term observation. Serum IV-7S levels in CR significantly decreased after IFN therapy and further decreased over long-term observation. Serum IV-7S levels over long-term observation were significantly lower than those at the end of IFN therapy in CR and PR and significantly lower than the initial values in all three groups of patients. The progression of fibrosis was not significant over long-term observation in liver biopsy specimens of 10 patients. The results of the present study suggest that high-dose IFN-alpha therapy for 6 months suppresses the progression of hepatic fibrosis for more than 5 years not only in CR but also in PR.