Flow cytometric analysis of the ploidy of normal and neoplastic hepatocyte nuclei obtained from adult woodchucks, a model of human hepadnavirus-induced hepatocellular carcinoma, was performed. All 36 samples of nuclei from non-neoplastic liver from woodchuck hepatitis virus-infected or uninfected liver were diploid, indicating that age-related nuclear polyploidization does not occur in this species, unlike other rodents. Individual or multiple hepatic neoplasms were obtained from each of 14 woodchuck hepatitis virus-infected woodchucks. Nineteen samples of hepatocellular carcinoma and eight adenomas were examined. Aneuploid nuclei were detected in 10 of the hepatocellular carcinomas and three of the adenoma samples. Similar DNA indexes, ranging from 1.11 to 1.22, were found in 7 of the 10 aneuploid HCCs and all 3 aneuploid adenomas. Nine of the 19 hepatocellular carcinoma samples and 5 of the 8 adenomas were diploid. Four of the diploid hepatocellular carcinomas had increased proportions of tetraploid nuclei. The presence of aneuploid nuclei was not related to histological appearance of the neoplasms or serum gamma-glutamyltranspeptidase levels. Because none of the hepatocellular carcinomas metastasized, the presence of aneuploidy could not be related to biological behavior. We determined the proportion of uninucleate and binucleate hepatocytes in hepatocellular carcinoma and nonneoplastic liver. Approximately 7% of hepatocytes were binucleate in nonneoplastic liver from woodchuck hepatitis virus-infected and uninfected liver. Only 2% of malignant hepatocytes were binucleate. The results of this study indicate that aneuploidy is a common change in hepatic neoplasms from woodchuck hepatitis virus-infected woodchucks.