This is the first report in the English literature of a composite endometrial tumor composed of papillary serous carcinoma and small cell carcinoma. A 79-year-old woman underwent total abdominal hysterectomy and left salpingo-oophorectomy due to endometrial carcinoma. Grossly, the uterus was enlarged with an irregular and nodular serosal surface, thickened myometrium, and irregular endometrium. Microscopic examination revealed an endometrial carcinoma composed of papillary serous carcinoma and small cell carcinoma. There was a differential immunoreactivity between the two components: the cells of the papillary serous carcinoma were positive for cytokeratin, CA-125, CEA, and HER-2/Neu, whereas these markers were negative in the small cell carcinoma. Various neuroendocrine markers were positive in the small cell carcinoma and negative in the papillary serous carcinoma. Fluorescence in situ hybridization analysis using 4, 8, and 10 centromeric probes revealed hyperploidy (6-8 signals) in the small cell carcinoma cells. Most of the serous carcinoma cells were euploid, with scattered trisomies and tetrasomies of these chromosomes. The patient died of progressive disease 5 months after surgery. We suggest that the small cell carcinoma may have arisen from the endometrial papillary serous carcinoma undergoing tumor progression with neuroendocrine differentiation.