The amplification and/or rearrangement of the cyclin D1 gene, a positive regulatory element of the G1 to S phase of the cell cycle, has been reported in various human tumors, suggesting an oncogenic role of this gene. In this study, we investigated the expression of cyclin D1 in the formalin-fixed and paraffin-embedded human hepatocellular carcinoma (HCC) tissues of 25 patients, using monoclonal antibody 5D4 raised against cyclin D1. Two distinct patterns of staining were observed in HCC cells, nuclear and cytoplasmic. The nuclear staining pattern of cyclin D1 was detected in the tissues of only 2 of the 25 HCC patients (8%) examined and no particular clinicopathological characteristics were found in these patients. In contrast, the cytoplasmic staining pattern, without nuclear staining, was detected in 8 of the 25 patients with HCC (32%). A significant correlation was found between the expression of cytoplasmic cyclin D1 and patients with tumor thrombus in the portal vein (Vp), as well as those with intrahepatic metastasis (IM). These results indicate that the cytoplasmic cyclin D1 expression appears to be related to the prognosis of HCC. The Ag nucleolar organizer regions (NORs) counts in cyclin D1-positive and -negative patients were not significantly different, suggesting that immunostaining for cyclin D1 has the potential to be a unique prognostic marker in human HCC. Simultaneous immunohistochemical study with p53 antibody in the same series of HCC revealed that 88% of the patients positive for cyclin D1 also expressed p53 and that in 91% of the patients negative for p53, cyclin D1 was not expressed. These results suggest that cyclin D1 is expressed later than the alteration of p53 in the progression of human HCC.