Epithelial ovarian tumors are responsive to steroid hormone stimulation and the ovarian stroma may have a direct role in this process. We evaluated immunohistochemical markers of sex-steroid differentiation and steroidogenesis (calretinin, inhibin, steroidogenic factor 1), steroid enzymes involved in hormone biosynthesis (CYP17, CYP19, HSD17β1, AKR1C3), and hormone receptors (estrogen receptor, progesterone receptor, and androgen receptor) in 101 epithelial ovarian tumors and in normal structures implicated in ovarian carcinogenesis (ovarian surface epithelium and cortical inclusion cysts) in an attempt to elucidate this process. We hypothesized that ovarian stroma immediately adjacent to tumors express markers of sex-steroid differentiation and steroidogenesis and steroid enzymes whereas the epithelium contains corresponding hormone receptors. As the findings in seromucinous, endometrioid, and clear cell neoplasms, tumors closely associated with endometriosis, were very similar, these were combined into a group designated 'endometriosis-related tumors.' Significantly increased expression of markers of sex-steroid differentiation and steroidogenesis was found in stroma immediately adjacent to endometriosis-related tumors (P=0.003) and mucinous tumors (primary and metastatic mucinous tumors were combined because of similar findings) (P<0.0001) compared with more remote ovarian stroma. In addition, sex-steroid enzymes were increased in stroma adjacent to endometriosis-related tumors (P=0.02) and mucinous tumors (P=0.02) compared with more distant stroma. Steroid hormone receptors showed greater expression in epithelium compared with stroma in the endometriosis-related tumors (P=0.0009), low-grade serous tumors (P<0.0001), and high-grade serous carcinoma (P=0.0036). In contrast, there was greater expression in stroma compared with epithelium (P<0.0001) in mucinous tumors, which may be due to the fact that they are not derived from müllerian epithelium. In conclusion, our findings strongly support the view that the stroma surrounding epithelial tumors in the ovary is activated to elaborate steroid hormones which may stimulate further neoplastic growth. The precise mechanisms by which this process might occur are complex and require further investigation.