Extrapulmonary small cell carcinomas are often associated with carcinomas of other cell types. Although a hypothesis that extrapulmonary small cell carcinomas arise from a multipotential stem cell could explain this mixed feature, recent molecular evidence supports another possibility that the small cell component may arise as a late-stage phenomenon in the progression of more organ-typical carcinomas. Here, we report a case of uterine cervical adenocarcinoma containing 30% of small cell carcinoma. Adenocarcinoma was located in the endometrial side of the tumor that was adjacent to the normal cervical region, while small cell carcinoma was located in the periphery of the tumor. The transition from adenocarcinoma to small cell carcinoma was observed in the boundary area. These findings suggest that cervical small cell carcinoma can be differentiated from pre-existing adenocarcinoma and offer further support to the hypothesis that the small cell component arises as a late-stage phenomenon in the progression of more organ-typical carcinomas.