The case of a biliary cystadenoma containing foci of cystadenocarcinoma is reported, along with the first electron microscopic description of this lesion. Both neoplastic and congenital origins have been implicated in the pathogenesis of biliary cystadenomas. In this case hamartomatous bile ducts were found, supporting a congenital origin for at least some biliary cystadenomas. Careful pathologic evaluation is emphasized since malignant degeneration can be detected only after thorough sectioning.