Two cases of sclerosing stromal tumor of the ovary (SST) are described. Immunohistochemical analysis of SSTs was performed using formalin-fixed and paraffin-embedded materials in comparison with thecofibromas and normal ovaries. The existence of desmin- and smooth-muscle actin-positive isolated spindle cells in SSTs and also in cortical stroma of the normal ovary was characteristically demonstrated, whereas such smooth-muscle-like cells were absent from six thecofibromas. Immunohistochemistry of desmin and smooth-muscle actin is useful in distinguishing SST from thecofibroma. Ultrastructural study further evidenced the presence of smooth-muscle-like cells in SSTs. In addition, we histochemically demonstrated heterogeneous cellular components in SSTs, including fibroblastic, thecalike, smooth muscle and endothelial cells. Hence, it is suggested that SST is derived from pluripotent immature stromal cells of the ovarian cortex.