The aim of this study was to investigate serum prolidase enzyme activity and to find out its association with liver biopsy specimens' histopathological findings in patients with nonalcoholic steatohepatitis (NASH), which may progress to liver fibrosis and cirrhosis. Thirty‐six patients with biopsy‐proven NASH and 29 healthy controls were enrolled. Serum prolidase enzyme activity was measured spectrophotometrically. Serum prolidase enzyme activity was significantly higher in patients with NASH thancontrols ( P=0.016). A significant correlation was observed between serum prolidase enzyme activity and fibrosis score in patients with NASH ( r=0.661, P<0.001). Serum prolidase activity seems to be correlated with the level of fibrosis. Monitoring serum prolidase activity may be a useful adjunctive tool in predicting liver fibrosis, especially in the absence of advanced fibrosis and other conditions, which may affect the interpretation of prolidase activity. J. Clin. Lab. Anal. 24:207–211, 2010. © 2009 Wiley‐Liss, Inc.