Squamous cell carcinoma in situ arising in an ovarian mature cystic teratoma. Report of one case with histopathologic, cytogenetic, and flow cytometric DNA content analysis. A squamous cell carcinoma in situ arose in an ovarian mature teratoma (ie, dermoid cyst) in a 62-year-old woman. Flow cytometric DNA content analysis of paraffin-embedded in situ carcinoma showed a normal DNA content with moderate to high proliferative activity (S-phase fraction estimate, 16% to 18%). Cytogenetic analysis of the in situ cancer and the benign cystic portion of the tumor revealed a 46,XX karyotype. In addition, the benign cystic portion of the tumor revealed homozygous chromosomal heteromorphisms, compared with heterozygous markers found in peripheral blood lymphocytes. These results show that this squamous cell carcinoma in situ was euploid and suggest that the mature cystic teratoma was derived from a single germ cell after meiosis I. 