A peculiar primary ovarian neoplasm occurring in a 28-year-old woman is presented. The tumor cells showed extensive cytoplasmic vacuolation, resulting in a "signet-ring" appearance similar to what was described in Krukenberg tumors. The tumor was studied histochemically, as well as by light and electron microscopy. The cells appeared to be uniform. A moderately electron-dense flocculent material was present in the cytoplasmic vacuoles. similar material was seen in the intercellular matrix with some fibrillary structures suggesting a protein structure. This, coupled with the lack of any basement membrane material around the tumor cells, indicates a mesenchymal tumor that is probably derived from the multipotent ovarian stromal cell. The features that help to differentiate this tumor from other ovarian tumors with clear or vacuolated cells are discussed. Because of its gross and microscopic appearance, the tumor is expected to behave in a benign fashion.