We found, by reverse transcription--real time--polymerase chain reaction, that the expression of aromatase (CYP19) in ovarian, peritoneal endometriosis, and deep endometriotic nodules is significantly different, which strengthens the theory of three distinct clinical entities. Compared with peritoneal endometriosis, ovarian endometriosis exhibits an 8-fold higher expression of aromatase, which suggests that aromatase inhibitors may be particularly active in this form of endometriosis.