17Beta-hydroxysteroid dehydrogenase (17betaHSD) isoforms reversibly catalyze the final step in the formation of estradiol (E2) from estrone (E1) and the formation of testosterone from androstenedione. We have investigated 17betaHSD type 1, 2, 3, and 4 gene expression and 17betaHSD estrogenic activity in human anterior pituitary adenomas. 17BetaHSD messenger ribonucleic acid (mRNA) expression was studied by RT-PCR in 42 pituitary tumors and 3 normal pituitaries, 17betaHSD activity was studied in 11 tumors and 17betaHSD type 1 was immunolocalized in vitro in 6 tumors. 17BetaHSD type 1 gene expression was detected in 34 of 42 adenomas in all tumor subtypes; 17betaHSD type 2 mRNA was detected in 18 of 42 adenomas, but not in prolactinomas; 17betaHSD type 3 mRNA was detected in 12 of 42 adenomas, but not in corticotropinomas; 17betaHSD type 4 was expressed in 20 of 42 adenomas by all adenoma subtypes. Reversible 17betaHSD activity was found in 9 of 11 adenomas, and 17betaHSD type 1 immunopositivity was cytoplasmically distributed in all 6 adenomas in vitro. All 4 17betaHSD isoforms are variably expressed in human anterior pituitary adenomas, which also show 17betaHSD enzyme activity, suggesting that 17betaHSD may play an important role in regulating the local cellular levels of estradiol.