The expression of 5alpha-reductase types 1 and 2 was examined in human breast carcinoma using immunohistochemistry and RT-PCR. Immunoreactivity for 5alpha-reductase isozymes was also correlated with various clinicopathological parameters to examine possible local regulatory mechanisms of sex steroids, including progesterone and androgens, in human breast carcinoma tissues. Immunoreactivity for 5alpha-reductase type 1 was detected in the cytoplasm and possibly in the nuclear membrane of tumor cells in 35 of 60 invasive ductal carcinomas (58%), and type 2 signal was detected in 9 of these 60 cases (15%). The results from RT-PCR (n = 8) were consistent with those from immunohistochemistry. A significant positive correlation was detected between 5alpha-reductase type 1 immunoreactivity and androgen and progesterone receptor A or B labeling indexes, and immunoreactivities of 5alpha-reductase type 2, 17beta-hydroxysteroid dehydrogenase type 5, or 3beta-hydroxysteroid dehydrogenase, which recognizes both types I and II. An inverse correlation was detected between 5alpha-reductase type 1 immunoreactivity and tumor size, histological grade, or Ki-67 labeling index. 5alpha-Reductase type 2 immunoreactivity was significantly correlated with 17beta-hydroxysteroid dehydrogenase type 5 immunoreactivity, but not with other parameters. This study suggests that 5alpha-reductase type 1 is mainly expressed in human breast carcinoma, which may play an important role in the in situ production and actions of the potent androgen, 5alpha-dihydrotestosterone, including inhibition of cancer cell proliferation, in hormone-dependent human breast carcinoma.