BACKGROUND: Undifferentiated sarcoma of the liver is a rare tumor of childhood. The histology has been well documented, but the histogenesis is controversial. The cytologic characteristics of this neoplasm in aspiration material have not been previously documented. CASE: The cytologic features in a 9-year-old male were pleomorphic malignant cells, multinucleate giant cells, numerous hyaline globules and myxoid matrix. CONCLUSION: The cytologic features of undifferentiated sarcoma of the liver are distinctive and different from those of other childhood liver tumors. Awareness of the tumor's cytomorphology can result in a confident preoperative diagnosis.