The KISS1/KISS1R system has been implicated in the physiology of reproduction and many studies have documented the stimulatory effect of kisspeptin on Gonadotropin-releasing Hormone (GnRH) and gonadotropin secretion. In addition, the KISS1/KISS1R system has been implicated in several pathophysiological processes, including cancer.We examined the pattern of KISS1 and KISS1R expression in eutopic and ectopic endometrium tissues which were obtained from 24 women suffering from endometriosis and 16 control women who underwent laparoscopic excision for other benign gynecological diseases.Significant KISS1R expression was detected in 10 out of the 24 samples of eutopic endometrial biopsies of women suffering from endometriosis, while their matched biopsies of ectopic endometrial lesions did not reveal any KISS1R expression. KISS1R expression was not detected in the endometrial biopsies of control women. In addition, KISS1 expression was not detected in practically any the endometrial tissues of either control women or women with endometriosis.The expression of KISS1R in 10/24 samples of human endometrial biopsies of women suffering from endometriosis and the loss of its expression in the samples of matched ectopic endometrial tissues, suggests that the KISS1/KISS1R system may play a role in the pathophysiology of endometriosis only for a particular group of patients. Since KISS1 is not expressed by the endometrium and endometriotic tissue, it is conceivable that the activation of KISS1R in this particular group is mediated by KISS1 expression by non-endometrial tissues (endocrine action).