A case of an unusual type of hepatoblastoma in a 5-month-old male infant is described. The tumor showed the following unusual features as compared with the epithelial and mixed types of hepatoblastoma: (1) The tumor cells presented a primitive anaplastic appearance without any resemblance in terms of cytologic features or arrangement to embryonal or fetal liver. (2) Abundant mucoid material containing acid mucopolysaccharide was present giving a slimy, gelatinous, gross appearance to the tumor and its metastases. (3) Well-defined tubular structures were present in some foci within the tumor parenchyma. (4) The tumor resulted in a rapidly fatal course, with metastases to the lungs and widespread peritoneal seeding despite complete surgical resection of the primary tumor. Yolk sac carcinoma and undifferentiated (embryonal) sarcoma of the liver were considered in the differential diagnosis. On electron microscopic examination, the tumor cells showed cytoplasmic features and junctional complexes consistent with their epithelial origin. Alpha-fetoprotein in the blood, which was markedly elevated prior to surgery, returned to a normal level postoperatively. Hepatoblastoma with the combination of features described above has not been previously reported. Because of the two striking and easily recognizable features viz. total lack of differentiation of tumor cells and presence of abundant mucoid material, the authors designated the tumor as mucoid anaplastic hepatoblastoma.