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We report a case of massive intra-abdominal undifferentiated carcinoma derived from a tiny well-differentiated endometrioid adenocarcinoma of the ovary. The patient, a 56-year-old woman, who presented with a large intra-abdominal mass, underwent cytoreductive surgery with hysterectomy and bilateral salpingo-oophorectomy. Macroscopically, the intra-abdominal mass was composed of fragile and solid tumor components with extensive necro-hemorrhagic areas, mimicking a primary peritoneal tumor. Both ovaries were apparently normal in size, but a cut section of the right ovary revealed a 2-cm solid and cystic tumor showing focal rupture to the peritoneal surface. The intra-abdominal tumor consisted of pleomorphic cells without specific differentiation, showing diffuse sheet-like proliferation. The right ovarian tumor was a histologically well-differentiated endometrioid adenocarcinoma. Both the intra-abdominal undifferentiated tumor and the ovarian adenocarcinoma cells were immunohistochemically positive for keratin AE1/3, Ber-EP4, and CD10. Epithelial membrane antigen was positive only in the ovarian adenocarcinoma component, and vimentin was diffusely positive only in the intra-abdominal undifferentiated tumor component. Calretinin was negative in both tumor components. Allelotype analysis using 24 polymorphic markers located on 12 chromosomal arms showed that the intra-abdominal undifferentiated carcinoma and ovarian adenocarcinoma components had a high concordance rate (88%) of allelic patterns including identical allelic loss patterns at 7 chromosomal loci, suggesting a common genetic lineage. These data suggest that ovarian endometrioid adenocarcinoma, even when small in size, can give rise to a massive undifferentiated carcinoma filling the peritoneal cavity.