Background. We evaluated the annual rate of fibrosis progression in chronic hepatitis B and C patients with elevated alanine aminotransferase (ALT) levels. Methods. Forty-nine chronic hepatitis B patients and 21 chronic hepatitis C patients, each of whom had undergone two or more liver biopsies at an interval of more than 1 year, were enrolled in this retrospective clinical research protocol. The annual rate of fibrosis progression was calculated by dividing the change in fibrosis stage between the first and second liver biopsies by the interval in years between them. Results. The median interval in chronic hepatitis B and C was 3.4 (first and third quartiles, 1.8-4.7) and 3.2 (2.1-6.5) years, respectively. Overall, the mean fibrosis progression rate was 0.21 +/- 0.31 (mean +/- SD) fibrosis units (FU) per year in 49 patients with chronic hepatitis B, and 0.13 +/- 0.18 FU/year in 21 patients with chronic hepatitis C. The ALT level was an independent variable correlating with fibrosis progression. In patients whose median ALT level was 70 IU/l or more, the mean fibrosis progression rate was 0.28 +/- 0.32 FU/year in 36 patients with chronic hepatitis B, and 0.22 +/- 0.23 FU/year in eight patients with chronic hepatitis C. Conclusion. This paired-biopsy study of untreated chronic hepatitis B or C demonstrated that fibrosis progression occurred largely in patients with continuously elevated ALT levels even over a relatively short period, and that liver fibrosis might progress by one stage within an average of 4-5 years of follow-up in patients with elevated ALT of 70 IU/l or more.