Undifferentiated embryonal sarcoma (UES) of the liver is a primitive mesenchymal, malignant neoplasm occurring in children. The link between UES and mesenchymal hamartoma (MH) is controversial. Whether they share the same histiogenesis, representing 2 ends of a spectrum, or are distinct entities is unclear. The genetic aberrations of these neoplasms are not well understood, although a common breakpoint (19q13.4) was recently identified. The purpose of this study was to elucidate immunohistochemical markers that may establish a link between the 2 tumors by reviewing cases of UES and MH. Cases of UES from 1990 to 2008 were identified. Clinical demographics were reviewed. Hematoxylin and eosin staining and immunohistochemical staining for vimentin, alpha-1 antitrypsin, and alpha-fetoprotein were performed. Eleven children were diagnosed with UES. Five cases were seen arising in association with MH, and transitional zones were evident. The mean age at presentation was 10 years. To our knowledge, the 11-month-old patient is the youngest reported case of UES in concurrence with MH. All UES tumor cells were positive for vimentin, diastase-resistant periodic acid–Schiff stain, and alpha-1 antitrypsin. Chromosomal analysis of 3 UES cases, 2 arising with MH, showed complex karyotypes with no involvement of 19q13.4. We suggest a continuum between UES and MH. Although a chromosomal anomaly of 19q13.4 was not identified, a submicroscopic involvement of this locus cannot be excluded. Additionally, our analyses suggest that multiple chromosomal aberrations may be associated with the MH/UES spectrum.