The aim of this study was to evaluate whether androgen receptor (AR) and the enzymes that convert testosterone into the more potent androgen dihydrotestosterone, 5alpha-reductases (5alpha-R1 and 5alpha-R2) are expressed in pelvic endometriosis. The study involved 21 infertile women who underwent laparoscopy and were divided into two groups: control (n= 13) and endometriosis (n= 8) according to the histological and laparoscopic findings. Endometrial and endometriotic implant biopsies were performed. By reverse transcription polymerase chain reaction and immunohistochemistry, AR, 5alpha-R1 and 5alpha-R2 messenger RNA and protein were detected in biopsies of pelvic endometriosis, as well as in the eutopic endometrium of both groups. These findings suggest that active androgens may be formed within the endometriotic tissue and that both local and systemic androgens have the potential to act on endometriotic cells.