The ultrastructural characteristics of fetal liver, two hepatoblastomas and two hepatocellular carcinomas were compared. Tumor cells of hepatoblastoma disclosed monotonous nuclei, poorly-developed cytoplasmic membrane system, abundant free ribosomes and prominent glycogen granules. Those of hepatocellular carcinoma revealed comparatively pleomorphic nuclei, welldeveloped cytoplasmic membrane system, a few free ribosomes and numerous glycogen granules. Fetal liver showed monotonous nuclei, well-developed RER, abundant free ribosomes and prominent glycogen granules. Young mesenchymal cells with well-developed RER and continuous basal lamina surrounding the epithelial cells were detected in both cases of hepatoblastoma but not in those of hepatocellular carcinoma. Tumor cells of hepatoblastoma in a case showed intramitochondrial crystalloids and thick bundles of fibrils in the cytoplasm. The tumor cells usually show a high level of alpha fetoprotein1'2 (AFP), a high nuclearcytoplasmic ratio and cytoplasmic basophilia. On the other hand, tumor cells of hepatocellular carcinoma are associated with a lower serum level of AFP than hepatoblastoma, a lower nuclear-cytoplasmic ratio and cytoplasmic eosinophilia.Two hepatoblastomas and two hepatocellular carcinomas, in addition to a fetal liver, were subjected to electron microscopy as well as light microscopy. This study was intended to clarify ultrastructural difference between these two malignancies. The clinical and pathologic features of the five cases are summarized in Table 1.