The present study was conducted to investigate whether GnRH-receptor (GnRH-R) gene is expressed in endometriosis ovarian implants and whether a GnRH-analogue (GnRH-a) may exert an effect on endometriosis cell proliferation in vitro. The presence of GnRH-R transcripts in ovarian endometriosis cells was assessed by reverse transcription-polymerase chain reaction (RT-PCR) and further confirmed by Southern blot analysis. GnRH-R mRNA was detected in all the 13 samples examined. In contrast, GnRH-R transcripts were not detectable in endometriosis-free peritoneal tissue. In the second part of the study, endometriosis cells were cultured for 9 days with different doses of leuprolide acetate (ranging from 0 to 10(-5) M). In 4 out of 13 cases, a significant anti-proliferative effect was observed at doses of leuprolide acetate ranging from 10(-9) to 10(-5) M. In one case, a significant inhibition of cell proliferation was observed only at 10(-5) M leuprolide acetate concentration. In contrast, the GnRH-a did not affect cell growth, regardless of the expression of GnRH-R transcripts and the given doses, in the remaining 8 experiments. To date, this is the first evidence indicating that GnRH-R mRNA is expressed in human ovarian endometriomas. Moreover, the inhibition of endometriosis cell proliferation induced by the GnRH-a in vitro suggests that, at least in some cases, this compound might exert a direct effect on endometriosis lesions.