Overexpression of cyclooxygenase 2 (COX-2) is associated with tumorigenesis in a number of human cancers. Recently, COX-2 overexpression has also been reported in hepatocellular carcinoma (HCC), especially in well-differentiated HCC. However, doubt has been cast on these claims concerning HCC. Here we show by Western blot analysis that COX-2 protein level is higher in the adjacent chronic hepatitis liver than in the tumors themselves. We also show, by immunohistochemical staining, that the mean intensity of COX-2 expression in cirrhotic liver specimens is significantly higher than in normal livers and in moderately-differentiated HCC. In addition, the frequency and level of expression of COX-2 in poorly differentiated HCC was similar to that of well-differentiated HCC. Nevertheless all types of HCC expressed more COX-2 than normal livers, and immunofluorescence staining showed cytoplasmic expression of COX-2 in 7 out of 8 human hepatoma cell lines. Collectively, our data suggest that both chemoprevention and chemotherapy of HCC by COX-2 specific inhibitors should be considered. Our data also suggest that COX-2 may play a role in the advanced stages as well as early stages of hepatocarcinogenesis.