Small cell carcinomas of the ovary, hypercalcemic type, are unilateral tumors occurring almost exclusively in young women. 60% of the cases are associated with hypercalcemia. Microscopic examination shows diffuse clusters of small cells and follicle-like spaces. Immunohistochemically, the tumor cells are positive for epithelial and mesenchymal markers, and ultrastructurally they contain an abundance of dilated rough endoplasmatic reticulum and numerous free ribosomes. By DNA-single cell photometry, the neoplastic cells show a diploid DNA-content.