OBJECTIVES: To define the incidence of fibrosis progression among hepatitis C virus (HCV)/HIV-co-infected adults, to assess whether HCV or HIV treatment alters the risk of progression, and to determine the utility of liver biopsy to predict future disease. DESIGN: This prospective cohort evaluated 184 HIV/HCV-co-infected individuals who had at least two liver biopsies (median interval 2.9 years). METHODS: Biopsies were scored according to the Ishak modified histological activity index scoring system by a single pathologist blind to biopsy sequence. Significant fibrosis progression was defined as an increase of at least two Ishak fibrosis units between the first and second liver biopsy. Logistic regression analysis was used to assess determinants of fibrosis progression. RESULTS: A total of 174 non-cirrhotic patients were eligible; the majority were African-American men undergoing HIV treatment. On initial biopsy, no or minimal fibrosis was identified in 136 patients (77%). Significant fibrosis progression occurred in 41 patients (24%). Measures of HIV disease and its treatment before and after initial biopsy were not significantly different in progressors and non-progressors. Fibrosis progression was not associated with HCV treatment, which was received by 37 patients (21%) but only three sustained HCV-RNA suppression. In adjusted analysis, only an elevated serum aspartate aminotransferase level between biopsies was associated with progression (odd ratio 3.4, 95% confidence interval 1.4-7.9). CONCLUSION: Over a 3-year interval, significant fibrosis progression can occur in co-infected individuals even if minimal disease was detected on initial biopsy. In this context, factors other than treatment for HIV or HCV modify the risk of fibrosis progression.