Malignant liver tumors of infancy are uncommon. In this report the histological and ultrastructural features of a congenital mixed hepatoblastoma in a female infant are described. The tumor was obtained by radical surgery at the eighth day of life. Histologically the mass was characterized by a mixture of variously differentiated hepatic tissue and mesenchymal tissue. The epithelial component is represented by liver cells of the embryonic and fetal type. Electron microscopical investigation revealed a changing organelle composition of the epithelial tumor cells showing a decreased number of cytoplasmic organelles in the cells of the embryonic type. At cellular junctions, however, these cells are connected by desmosome-like structures without lumina as well as empty canaliculi-developing structures of cholangiolar microvilli. At light microscopical level the former corresponded to pseudorosettes , the latter to so-called rosettes. Furthermore, in places sinus-like structures are encountered but in the absence of true sinusoidal channels with Disse's spaces. Some areas consisted of undifferentiated cells lacking cytoplasmic organelles and resembled a multipotential blastema. The ultrastructural findings suggest that hepatoblastoma may arise from an undifferentiated progenitor cell capable of various forms of differentiation.