Cytopathological characteristics of undifferentiated carcinoma of the ovary were examined and reviewed. Seven cases of undifferentiated carcinoma were encountered among 202 malignant ovarian neoplasms in the Saitama Cancer Center. Histologically, two cases contained a minor component of usual Müllerian carcinoma and five others were purely of solid architecture without differentiation. Nevertheless, six cases, four of which were totally undifferentiated histologically, showed cytological features characteristic of adenocarcinoma. The results indicate that even carcinomas histologically diagnosed as undifferentiated may have cytological features of glandular differentiation, which may be Müllerian in origin.