Hepatic embryonal (undifferentiated) sarcoma (ES) is a rare pediatric tumor occurring predominantly in the first decade of life, but a few examples of adult ES have also been described. Isolated ultrastructural reports describe contradictory lines of differentiation in these tumors. Four pediatric and 3 adult ES cases were studied ultrastructurally and features were correlated with morphology. Morphologically, tumors were composed of mixture of plump spindle cells and bizarre giant cells, showing abundant cytoplasmic eosinophilic globules. Ultrastructurally, the hallmark features in all cases included dilated RERs and secondary lysosomes with dense precipitates. Dilated mitochondria and mitochondrial-RER complexes were often seen. Other features included intracytoplasmic fat droplets, scant actin microfilaments, and focal glycogen pools. In summary, pediatric and adult ES show similar morphologic and ultrastructural features. Ultrastructurally, hepatic ES have distinctive findings, including dilated RER and electron-dense lysosomal precipitates, which correlate with the eosinophilic hyaline bodies seen microscopically. These findings suggest that ES are composed of fibroblastic, fibrohistiocytic, and undifferentiated cells. Other lines of differentiation were not identified.