Short-term chronic exposures of rats to furan were recently found by us to preferentially induce a unique liver lobe pattern of development of small intestinal metaplasia and subsequent cholangiofibrosis, being essentially localized to the caudate and right liver lobes (L. W. Elmore, and A. E. Sirica, Cancer Res., 51: 5752-5759, 1991). We now demonstrate the preferential development of primary hepatic adenocarcinomas exhibiting small intestine mucosal cell differentiation, which have arisen at 70 to 90% incidences from the right/caudate liver lobes of Fischer 344 adult male rats by 16 months after their receiving furan by gavage at a daily dose of 30 mg/kg of body weight, five times a week, for 9, 12, and 13 weeks, respectively. In contrast, the incidences of primary hepatocellular carcinomas that developed in the furan-treated rats ranged from 0 to 20%, with the two hepatocellular carcinomas observed to be originating from the median/left liver lobes. Twenty-six of 27 hepatic adenocarcinomas analyzed exhibited glands containing on average 30.2% goblet cells, 2.1% Paneth cells, and 0.5% serotonin-positive neuroendocrine cells. Phenotypically, the glandular epithelial cells of the furan-induced intestinal-type adenocarcinomas were immunohistochemically positive for cytokeratin 19, but exhibited a heterogeneous pattern of immunohistochemical staining for gamma-glutamyl transpeptidase and showed no detectable immunostaining for transforming growth factor alpha. In addition, many of the glandular structures within these primary hepatic adenocarcinomas showed evidence of basement membrane disruption, as demonstrated by both electron microscopy and immunohistochemical staining for basement membrane laminin. While these intestinal-type adenocarcinomas appeared to have spread intrahepatically, none showed evidence of extrahepatic metastases. However, six of eight randomly selected adenocarcinomas grew progressively and retained their intestinal pattern of differentiation following serial transplantation into the fat pads of young adult Fischer 344 recipient rats. In this study, we also observed one primary hepatic cholangiocarcinoma that was characterized by a more native biliary rather than intestinal-type of differentiation. Interestingly, this was the only primary liver cancer observed by us to exhibit extrahepatic metastasis. In conclusion, our current findings clearly indicate that the small intestinal metaplasia and subsequent cholangiofibrosis developing early in the right/caudate liver lobes of furan-treated rats do not simply reflect reactive changes, but strongly correlate with the high incidences of intestinal-type of primary hepatic adenocarcinoma that occurs in the right/caudate liver lobes of rats after long-term exposures to furan.