Combined hepatocellular-cholangiocarcinoma (CHC) forms a small but significant proportion of primary liver carcinomas. However, its diagnostic features are not well established, and this has possibly contributed to the variability in its reported clinical outcome in the literature. Many such tumors with features intermediate between hepatocellular carcinoma and cholangiocarcinoma (CC) may have been considered CC in the past based on positivity for "biliary differentiation" cytokeratins and the lack of availability of highly sensitive and specific hepatocellular markers. The utility of in situ hybridization for albumin mRNA, a recently available sensitive and specific hepatocellular marker, has not been reported in CHC. We investigated 27 CHCs with regard to their histomorphologic spectrum and association of these morphologies with immunohistochemical staining for different cytokeratins (CK7, CK19, and CK20; AE1; Cam 5.2), epithelial membrane antigen, polyclonal carcinoembryonic antigen and alpha-fetoprotein, and in situ hybridization for albumin mRNA. All 27 tumors contained areas morphologically intermediate between hepatocellular carcinoma and CC (transitional-type tumors), and in each case such areas formed at least 25% of the tumor. Nine (33%) tumors showed areas with "antler-like" morphology, a feature not previously described in CHC. Twenty-two of 23 tumors (96%) showed positive signals on in situ hybridization for albumin mRNA. Positivity for both hepatocellular (albumin mRNA) and biliary (keratin immunohistochemical profile) markers confirmed the light microscopic impression of biphenotypic differentiation in these tumors. Immunohistochemical positivity for all cytokeratins (except CK7) and epithelial membrane antigen, as well as the expression of albumin mRNA by in situ hybridization, did not show significant differences between hepatocellular carcinoma and CC-like areas. Based on the cytokeratin profile and results on polyclonal carcinoembryonic antigen/alpha-fetoprotein alone, many such tumors would be classified as CC. However, the positivity for albumin mRNA by in situ hybridization proves that such an interpretation would not have been accurate. Clinically, CHCs showed many differences from pure hepatocellular carcinoma, including the absence of cirrhosis (0 of 27), rarity of serum hepatitis B or C marker positivity (4 of 27), and normal to only mildly elevated serum alpha-fetoprotein levels (median 187 ng/mL). The tumor followed an aggressive clinical course, with overall 3-and 5-year survival rates of 30% and 18%, and in the resected cases of 38% and 24%, respectively.