AbstractPrevious in vitro studies demonstrated that bioactive androgen 5α‐dihydrotestosterone (DHT) exerted antiproliferative effects through an interaction with androgen receptor (AR) in breast carcinoma cells. However, AR status has not been examined in association with DHT concentration in breast carcinoma tissues, and significance of androgenic actions remains unclear in breast carcinomas. Therefore, in our study, we first examined intratumoral DHT concentrations in 38 breast carcinoma tissues using liquid chromatography/electrospray tandem mass spectrometry. Intratumoral DHT concentration was positively associated with 5α‐reductase type 1 (5αRed1), and negatively correlated with aromatase. We then examined clinical significance of AR and 5αRed1 status in 115 breast carcinoma tissues by immunohistochemistry. Breast carcinomas positive for both AR and 5αRed1 were inversely associated with tumor size or Ki‐67. These patients showed significant associations with a decreased risk of recurrence and improved prognosis for overall survival, and the AR / 5αRed1 status was demonstrated an independent prognostic factor. Moreover, we examined possible regulation of DHT production by aromatase in in vitro studies. DHT synthesis from androstenedione in MCF‐7 cells was significantly inhibited by coculture with aromatase‐positive stromal cells, which was significantly reversed by addition of aromatase inhibitor exemestane. These results suggest that intratumoral DHT concentration is mainly determined by 5αRed1 and aromatase in breast carcinoma tissues, and antiproliferative effect of DHT may primarily occur in the cases positive for both AR and 5αRed1. Aromatase inhibitors may be more effective in these patients, possibly due to increasing local DHT concentration with estrogen deprivation. © 2006 Wiley‐Liss, Inc.