From July 1972 through September 1984, 8 of 44 children diagnosed as having primary malignant hepatic tumors, who were treated at St. Jude Children's Research Hospital, had undifferentiated (embryonal) sarcoma (five patients) or rhabdomyosarcoma (three patients). The natural history and response to multimodal therapy of these rare tumors are described. The pathologic material was reviewed and evidence for the differentiating potential of undifferentiated (embryonal) sarcoma is presented. At diagnosis, disease was restricted to the right lobe of the liver in three patients, was bilobar in four patients, and extended from the left lobe into the diaphragm in one patient. Lung metastases were present in two patients at diagnosis. All three patients with rhabdomyosarcoma had intrahepatic lesions without involvement of the biliary tree. Survival ranged from 6 to 73 months from diagnosis (median, 19.5 months); two patients are surviving disease-free for 55+ and 73+ months, and one patient recently underwent resection of a recurrent pulmonary nodule 22 months from initial diagnosis. Three patients died of progressive intrahepatic and extrahepatic abdominal tumors, and two patients, who died of progressive pulmonary tumor, also had bone or brain metastasis but no recurrence of intra-abdominal tumor. Six patients had objective evidence of response to chemotherapy. The authors suggest an aggressive multimodal approach to the treatment of these rare tumors in children.