A sclerosing stromal ovarian tumor from a 15-year-old girl was studied. Enzyme histochemistry demonstrated the presence of 17 beta-estradiol, secondary alcohol, glucose-6-phosphate, and isocitrate dehydrogenase activities, strongly implicating that the tumor had steroidogenic capability. Electron microscopy revealed a diverse cellular population composed of conspicuous lipid-laden cells, fibroblast-like and undifferentiated primitive mesenchymal cells. The findings of the latter cell type and intermediate forms confirmed the stromal origin of sclerosing stromal tumor of the ovary.