Abstract. The ovaries contain cells that have the capacity for regeneration and cancer stem cells (CSC) that are capable of differentiating aberrantly from the homeostatic controls. The histology of ovarian cancer does not usually change in a patient. However, CSCs are the origin of a number of tumors. CSCs are known to exist in ovarian carcinomas and the expression of CD44, c-Kit and CD133 has been identified in such carcinomas. This study presents the case of a patient diagnosed with ovarian cancer with an abdominal mass who underwent surgery, eight cycles of gemcitabine-paclitaxel chemotherapy and irradiation. Pathological examination indicated a transformation from adenocarcinoma to undifferentiated small cell carcinoma. The expression of CD133 changed from negative to positive in ovarian carcinomas. The present case indicates that any histological changes observed in ovarian neoplasms originate from neoplastic stem cells. In addition, this case demonstrates the importance of repeatedly assessing therapy by tumor biopsy throughout the course of ovarian cancer treatment.