Although recurrent hepatitis C virus (HCV) after liver transplantation (LT) is universal, a minority of patients will develop cirrhosis within 5 years of surgery, which places them at risk for allograft failure. This retrospective study investigated whether 2 serum fibrosis markers, serum hyaluronic acid (HA) and YKL-40, could be used to predict rapid fibrosis progression (RFP) post-LT. These markers were compared with conventional laboratory tests, histological assessment, and hepatic stellate cell activity (HSCA), a key step in fibrogenesis, as assessed by immunohistochemical staining for alpha-smooth muscle actin. Serum and protocol liver biopsy samples were obtained from 46 LT recipients at means of 5 +/- 2 (biopsy 1) and 39 +/- 6 (biopsy 2) months post-LT, respectively. RFP was defined as an increase in the fibrosis score >or= 2 from biopsy 1 to biopsy 2 (a mean interval of 33 +/- 6 months). The ability of parameters at biopsy 1 to predict RFP was compared with the areas under receiver operating characteristic curves (AUROCs). Of the 46 subjects, 15 developed RFP. Serum HA and YKL-40 performed significantly better than conventional parameters and HSCA in predicting RFP post-LT for HCV at biopsy 1, with AUROCs of 0.89 and 0.92, respectively. The accuracy of serum HA >or= 90 microg/L and YKL-40 >or= 200 microg/L in predicting RFP at biopsy 1 was 80% and 96%, respectively. In conclusion, we found that elevated levels of serum HA and YKL-40 within the first 6 months after LT accurately predicted RFP. Larger studies evaluating the role of serum HA and YKL-40 in post-LT management are warranted.