We report the case of a hepatic undifferentiated (embryonal) sarcoma (UES) arising within a mesenchymal hamartoma (MH) in a 15-year-old girl. Mapping of the tumor demonstrated a typical MH transforming gradually into a UES composed of anaplastic stromal cells. When evaluated by flow cytometry, the MH was diploid and the UES showed a prominent aneuploid peak. Karyotypic analysis of the UES showed structural alterations of chromosome 19, which have been implicated as a potential genetic marker of MH. The histogenesis of MH and UES is still debated, and reports of a relationship between them, although suggested on the basis of histomorphologic similarities, have never been convincing. The histologic, flow cytometric, and cytogenetic evidence reported herein suggests a link between these two hepatic tumors of the pediatric population.