Hepatobiliary cystadenomas with mesenchymal stroma (CMS) are rare tumors. Two cases are reported that illustrate many features of CMS, including polypoid involvement of the common hepatic duct in one case. Both tumors were associated with elevated serum levels of the tumor-associated antigen CA 19-9 but normal levels of carcinoembryonic antigen and alpha-fetoprotein. Immunohistochemical analysis confirmed the presence of CA 19-9 in the epithelial component of the tumor. The implications of these findings are discussed, both in relation to the histogenesis of CMS and its preoperative diagnosis. A minimally elevated level of CA 19-9 was found in only one of five patients with hydatid disease of the liver, an important differential diagnosis in clinical management.