Type 1 17 beta-hydroxysteroid dehydrogenase (17 beta-HSD) is an estrogen-metabolizing enzyme that catalyzes the conversion of estrone to the more biologically potent estradiol. We examined the immunolocalization of 17 beta-HSD in five specimens of normal human ovary and in 51 specimens of common epithelial tumors of the ovary to study the expression of 17 beta-HSD in these ovarian neoplasms. 17 beta-HSD immunoreactivity was detected in granulosa cells of dominant preantral follicles but not in the surface epithelium of the normal cycling human ovary. In 12 patients, the enzyme was not expressed in tumor cells of benign cystadenoma. 17 beta-HSD was immunolocalized in the cytoplasm of four specimens (50%) of eight of tumors of low malignant potential and in 20 specimens (67%) of 30 of invasive carcinoma. 17 beta-HSD immunoreactivity was also observed in nonluteinized stromal cells of mucinous tumors (two adenomas of six, three tumors of five with low malignant potential, and two of 10 invasive carcinomas) but not in other histologic types. Type 1 17 beta-HSD expression, therefore, correlated with malignant transformation of the surface epithelium of the human ovary, supporting a possible role in its progression by increasing in situ estradiol concentration.