We describe an uncommon case of adenosquamous carcinoma arising in a Barrett esophagus in a 72-year-old white man who occasionally used alcohol, and was a nonsmoker for 34 years. Polymerase chain reaction-based microsatellite analysis was performed on the adenocarcinoma component (AC) and squamous cell carcinoma component (SC) of the tumor. The metaplastic Barrett epithelium (BE), the AC and the SC all showed loss of the same allele at 4 markers on chromosome 9p. Furthermore, the AC and the SC both showed loss of the same allele at all informative markers tested on chromosomal arms 3p, 5q, 10q, 14q, and 18q. In addition, both the SC and AC component contained the same missense mutation in the p53 tumor-suppressor gene. The only observed difference was a shift at a marker on chromosome 16q in the AC, whereas no shift was found in the BE and the SC. These findings suggest that this biphasic tumor has a monoclonal origin. The divergence presumably occurred late in the tumorigenesis of this carcinoma.