In estrogen metabolism, the enzymatic properties of the 17β- hydroxysteroid dehydrogenase (17βHSD) isozymes play very important roles in steroid hormone metabolism in various tissues, including the placenta. 17βHSD type 1 catalyzes primarily the reduction of estrone (E1) to estradiol (E2), whereas 17βHSD type 2 catalyzes primarily the oxidation er E2 to E1. In this study, we examined immunohistochemical localization of 17βHSD types 1 and 2 in human placenta (31 eases) ranging from 4-40 weeks gestation. The immunoreactivity of 17β3HSD type 1 was exclusively detected in syncytiotrophoblast from 4 weeks gestation to term placenta. Immunoreactivity of 17βHSD type 2 first appeared in endothelial celts of intravillous vessels at 12 weeks gestation, and the number of 17βHSD type 2- positive endothelial cells markedly increased up to 19 weeks, then reached a plateau. We quantitatively evaluated the 17βHSD type 2-positive endothelial cells in chorionic villi and determined the ratio of 17βHSD type 2-positive endothelial cells using immunohistochemistry of CD34, an endothelial antigen, in serial mirror tissue sections and subsequent image analysis using CAS 200. CD34 was detected from 4 weeks gestation, and its positive areas continued to increase toward term. The 17βHSD type 2-positive area per CD34-positive area markedly increased from 13 weeks gestation and reached a plateau at 19 weeks gestation, in which almost all endothelial cells were positive for 17βHSD type 2. 17βHSD type 2, therefore, is considered to prevent the passage of excessive estrogens into the fetal circulation at endothelial cells of the intravillous fetal capillaries by catalyzing the inactivation of E2 to E1.