Cyclin D1 is frequently overexpressed in hepatocellular carcinoma (HCC) exhibiting increased malignant phenotypes. It has also been known that the hepatitis Bx (HBx) protein is strongly associated with HCC development and progression. Although overexpression of both proteins is related to HCC, the relationship between the two has not been well studied. Here we show that HBx up-regulates cyclin D1 and that this process is mediated by the NF-kappaB2(p52)/BCL-3 complex. Our experiments indicate that HBx up-regulates BCL-3 in the mRNA level, which subsequently results in the up-regulation of the NF-kappaB2(p52)/BCL-3 complex in the nucleus. Moreover, impaired HBx-mediated BCL-3 up-regulation by small interfering RNA for BCL-3 reduced HBx-mediated cyclin D1 up-regulation. Down-regulation of the HBx protein level by p53 also reduced HBx-mediated cyclin D1 up-regulation. From these results, we conclude that the up-regulation of cyclin D1 by HBx is mediated by the up-regulation of NF-kappaB2(p52)/BCL-3 in the nucleus. This HBx-mediated-cyclin D1 up-regulation might play an important role in the HBx-mediated HCC development and progression.