The ultrastructural and immunohistochemical features of 19 hepatoblastomas were examined to evaluate the phenotypic expressivity of this solid embryonic neoplasm of childhood. Electron microscopy confirmed the embryonal and fetal characteristics of the neoplastic hepatocytes, but in addition, cells with features intermediate between these two cell types were identified. Dense bundles of collagen corresponding to the osteoid-like material by light microscopy surrounded nests of cells; the cells within this matrix stained for epithelial membrane antigen and vimentin and focally for cytokeratin, and they showed ultrastructural features of epithelial cells. The two cases of small cell hepatoblastoma reacted positively for vimentin and cytokeratin; the remaining 17 cases were immunoreactive for cytokeratin and α-fetoprotein, and some also for α1-antitrypsin, ferritin, and vimentin. A histogenetic scheme based on our findings is proposed to explain the divergent morphologic features of this neoplasm.