The serum level of hepatitis C virus (HCV)-RNA is clinically important as a predictor of the response to interferon (IFN) therapy in patients with chronic hepatitis C. If serum HCV-RNA levels fluctuate during follow-up, and IFN therapy is begun at the time of a low HCV-RNA level, the IFN therapy may be more effective. We evaluated the fluctuation of HCV-RNA serum levels for 2 years in 212 patients with chronic hepatitis C, untreated with IFN who had HCV genotype 1b and an HCV-RNA level of 10 Meq/ml or more at first consultation. The HCV-RNA level was measured monthly for 2 years with an HCV branched DNA probe assay (b DNA probe assay). We classified HCV-RNA patterns into three types by the ratio of maximum HCV-RNA level (a) to minimum HCV-RNA level (b). In pattern 1 (constant type, 151 patients; 71.2%) the a/ b ratio was 1-5. In pattern 2 (slight fluctuation type, 46 patients; 21.7%) the a/b ratio was 5-10. In pattern 3 (severe fluctuation type, 15 patients; 7.1%), the a/b ratio was 10 or more. Next, we evaluated the factors associated with the three patterns. Acute exacerbation of chronic hepatitis was regarded as an increase in serum alanine aminotransferase (ALT) level to more than 250 IU/l. The incidence of acute exacerbation for a 2-year follow-up was 13.9% (21/151) in pattern 1, 19.6% (9/46) in pattern 2, and 53.3% (8/15) in pattern 3. Multivariate analysis showed that acute exacerbation was the most important factor in the manifestation pattern 3. In conclusion, we found that: (1) about 70% of patients had a constant HCV-RNA levels for 2 years. (2) A few patients had severe fluctuation of serum HCV-RNA level after acute exacerbation of chronic hepatitis.