Undifferentiated (embryonal) sarcoma (UES) of the liver is a primary malignant tumor, rarely diagnosed in adults. Because of the absence of specific symptoms, rapid tumor growth, and normality of the common tumor markers, this neoplasm has a poor prognosis. Histologically, UES of the liver is characterized by anaplastic cells within myxoid matrix. Histological, immunohistochemical and chromosomic alterations are similar in UES and in mesenchymal hamartoma, suggesting a relation between these entities. The mainstay of treatment is surgery, while adjuvant treatment could increase survival.